[T-Mobile Samsung Galaxy S II SGH-T989] RealPariah:
I agree with the previous reply you are getting great speeds [I am in the Philly Metro Area] and we are a testbed and at full tilt I concur I have not seen more than 20 - 25 MBps and not 45 MBps as the ceiling. Keep in mind there are not many radios out there, really the VK1 is the updated radio that was part of the 2.3.6 update Samsung pushed to our phones. I have heard of the skyrocket radio working on ours but not the others [reliably].

ALSO, 4G IS NOT THE SAME AROUND THE WORLD IT IS LIKE SAYING ALL ANDROID PHONES WILL RUN THE SAME THINGS. Here in the US we run on a totally different frequencies than most of the rest of the world AND on top of that even some areas with the same carriers run on different freqs like TMo washington State runs on 700MHz and 1900 MHz while in my area it is 700 MHz only [I believe dont nit pick the statements as I am trying to remember these on the fly].

Due to borard restrictions I can not post outside links but go to google and search GSM World Coverage Map and first link should be from a worldtimezone

domain. This will highlight what I am getting at and give you an idea of what is going on with GSM 4G.

All of that to say be VERY careful which radios you use even if it does work with the phone your areas cell towers may not use compatible freqs.

Last thing just to be sure you are aware not every Galaxy S2 is an SGH-T989 that is only for the Tmobile Galaxy S2

Galaxy model List:

Tmo: SGH-T989

The T-Mobile USA version was codenamed the Samsung "Hercules" and given the model number SGH-T989 (as opposed to other phones with the Galaxy S II name which are I9100 models)

Sprint: Samsung Galaxy S II Epic 4G Touch (Model SPH-D710)

The Sprint version of the device, previously codenamed "Within", is known as the Samsung Galaxy S II Epic 4G Touch (Model SPH-D710). Unlike its predecessor, the Epic 4G, the Epic 4G Touch lacks a physical QWERTY keyboard. At 4.52 inches, it has a larger display than the international version. Additionally, the Epic 4G Touch features four touch-capacitive buttons, as opposed to the hardware/capacitive button combination found on the international version. Other differences include an LED notification light, larger battery (6.66Wh), and a WiMax radio. The device was launched on September 16, 2011. Unlike the T-Mobile USA and AT&T versions this particular model does not come with the NFC chip.

ATT: SGH-I777

The AT&T version was codenamed "Attain" (Model SGH-I777). However, it appeared on AT&T and Samsung websites simply as the Samsung Galaxy S II. It maintains a 4.3 inch screen, but also features four touch buttons. It also contains NFC. It was released on October 2, 2011.

SECOND ATT VARIANT [SAME PHONE]: Model SGH-I727

This is the second variant of this phone to come to AT&T (Model SGH-I727) along with some changes. For instance, the display features the same 4.52 inch screen shared with the Sprint model. Additionally, this variant also includes LTE radio, to coincide with AT&T's launch of its LTE network. However, to make this change it was required for the SoC chipset to be changed from the Exynos to the Qualcomm Snapdragon MSM8260, as the Exynos SoC does not support LTE (akin to a similar limitation made on the T-Mobile model). 'Skyrocket' was released on November 6, 2011 alongside a similar LTE-capable [HTC Vivid] at the same date. The NFC feature is not enabled.

US Cellular: U.S. Cellular's variant (Model SCH-R760) is the same as Sprint's Epic 4G Touch, but without the integrated WiMax radio. All other specifications remain the same

JAPANESE GALAXY S2 MODELS

In Japan, the Galaxy S2 is known as the NTT DoCoMo SC-02C

SOUTH KOREAN VARIANTS:

All of south Korean variants have a T-DMB tuner in place of FM radio tuner

SK Telecom's Galaxy S II (SHW-M250S)

The SK Telecom variant uses SK-MMS system instead of OMA-MMS system for MMS.

KT's Galaxy S II (SHW-M250K)

This variant uses KT's Wi-Fi CM instead of Android's Wi-Fi CM

LG U+'s Galaxy S II (SHW-M250L)

Because of mobile networks LG U+ have, it uses EV-DO Rev.B (KPCS 1.8Ghz) instead of WCDMA and HSPA. It is slightly thicker (9.4mm) than SK Telecom and KT variants (8.89mm).

CANADIAN VARIANTS

Rogers' Galaxy S II LTE (SGH-i727R)

This phone features a larger screen 4.52", a bigger battery 1850mAh, and a different 1.5GHZ Qualcomm processor. Rogers will carry the Samsung Galaxy S II LTE, launching in Fall 2011, soon after its LTE Launch in Toronto.[89]

Bell's Galaxy S II (GT-I9100M)

Bell's Samsung Galaxy S II is identical to the international version except that its model number is I9100M.

Telus' 4G Galaxy S II X (SGH-T989D)

Telus' Galaxy S II is virtually identical to the T-Mobile variant. It has a Qualcomm 1.5 Ghz dual core processor, larger 4.52 inch screen and 1850 mAh battery, is thicker at 9.4mm and has a different design.[90] There is a chrome band around the edge and the plastic on the back has a leathery feel.[91] Instead of the hardware home button, it has the standard four capacitive buttons. The Qualcomm processor allows for 42Mbps HSPA+ download speeds that the Samsung Exynos processor is not currently capable of. It was released on October 28, 2011.

[hopefully saved those the time of looking up the details of at least the US models [some info found on international versions but most info found on US variants per Wikipedia and again due to not allowed to post external links yet hope this helps a little [still confusing as hell all things considered:

So as you can see EVEN THOUGH THEY ARE ALL KNOWN AS SAMSUNG GALAXY S2s there are variables to consider in a myriad of different ways. Now some are compatible with others [I BELIEVE US Tmo variants work with the Candian variant [note the same 1.5 DC CPU [this is one of the bigger pieces as CPU instructions dictate a lot of how the code is written for a ROM].

OH AND please dont think I am saying you cant play with the modem drivers just giving you some pointers on how to better determine which will work best for you AND STRESSING THAT YOU RUN A FULL NANDROID BACKUP and store it to a safe place that is not any form of phone storage [that way you know even if you accidentally format something you didnt just shoot yourself in the foot].

Hope that helps!