[G2 and Desire Z Android Development] kusuriurikun:
In regards to the data speed tweaks in build.prop--it seems the munging in that may be a little deeper, with some invalid values in the original patch.

Now that I've found some good documentation (including in the Cyanogenmod forums and here on XDA) and know what I'm mucking with :D, here's what the relevant section of build.prop should look like:

# These are modified based on documentation in the CyanogenMod forums

# (http://forum.cyanogenmod.com/topic/9781-buildprop-roril-meanings/)

# and documentation on GPRS classes and coding schemes (listed at

# http://www.cellular-news.com/gprs/GPRS_classes_and_coding_schemes.shtml)

# to improve performance. The original patch would break GPS and had

# some nonfunctional values; this has hopefully been fixed.

#

# Mods and notes as follows:

#

# A53 enable has not been altered. Has to do with GPS encryption; not

# in stock build.prop for the Vision.

#

# Enable DTM set to 1; per the Cyanogenmod build.prop documentation

# forum this is a switch to allow voice and data on the same radio channel.

# Per the Vision RF team prefs, in theory this should be settable to 1,

# Enabled in the stock Vision ROM, so enabled here.

#

# GPRS class changed to 12 (5 slots:32-48kbps U/D) from 10 (5 slots:

# 16-24kbps U/32-48 kbps D)

# HSDPA category changed to 14 (21.1 Mbit/s) from 8 (7.2 Mbit/s)

# HSUPA category changed to 7 (11.5 Mbit/s) from 6 (5.76 Mbit/s)

# Didn't touch HSXPA as this is already optimised (setting 2 is HSDPA/HSUPA;

# I have found no documentation on setting 3 and this breaks GPS, setting

# 2 enables HSDPA on T-Mobile US; most docs on setting 3 are for AT&T and

# may be a flag for HSUPA on AT&T specifically. Default on stock build.prop

# is HSXPA = 2.)

#

# Needless to say, these are tweaked higher than the standard build.prop

# for the Vision. I've attempted to keep these sane but with noticable

# performance improvements and without the bogus values in the last patch.

#

# Haven't messed with the prefix junk; no docs on what they do. Not in

# stock build prop for Vision.

#

# Haven't messed with Enable SDR or Enable GEA3 because I haven't found

# solid documentation on what these do. Not in stock build.prop for Vision.

#

# Please report performance or issues with connectivity -kusuriurikun

ro.ril.enable.a53=1

ro.ril.enable.dtm=1

ro.ril.gprsclass=12

ro.ril.hsdpa.category=14

ro.ril.hsupa.category=7

ro.ril.hsxpa=2

ro.ril.def.agps.feature=1

ro.ril.disable.fd.plmn.prefix=23402,23410,23411

ro.ril.enable.sdr=0

ro.ril.enable.gea3=0

# vision RF team request

# Modded for speed (and based on the build.prop in stock Vision ROMs)

# Mods as follows:

# GPRS class unchanged

# HSDPA category changed to 14 (21.1 Mbit/s) from 8 (7.2 Mbit/s)

# HSUPA category changed to 7 (11.5 Mbit/s) from 6 (5.76 Mbit/s)

# Didn't touch HSXPA as this is already optimised.

# Per Cyanogenmod this is what the phone should be using (per

# http://code.google.com/p/cyanogenmod/issues/detail?id=2837); if

# the settings above here cause issues, comment 'em out. Again,

# tweaked from stock build.prop. -kusuriurikun

ro.ril.enable.dtm = 1

ro.ril.gprsclass = 12

ro.ril.hsdpa.category = 14

ro.ril.hsupa.category = 7

ro.ril.hsxpa = 2

In fact, should Tsubus desire, I've attached an experimental modded build.prop file for developing a patch :D (Obviously, rename to build.prop from build.prop.modded.txt)

And yes, I'll be eating my own dogfood in this, we'll see what performance improvements I get out of this one. :D

EDIT: OK, now that dogfood has been tried... :D

With the "Speedy Gonzales Tweak", I'm seeing marginal improvement with downloads, marked improvement in UPLOAD speeds. (At most I see maybe 1 MB/s increased d/l speeds in my area with the mod (from about 3000KB/s to 4500 KB/s), but upload speeds improve considerably (from 300 KB/s to around 692 KB/s; I should note this was also done at a time when there's network congestion in my area).

This is not a HUGE improvement, but it does help for those who need it. :D And, happily, I can confirm this version DOESN'T break GPS. :D

(As for me--I pretty much went right back to the stock one, but then again I'm not the world's heaviest data user on my phone; that's what wifi hotspots are for. :D In fact, part of why I did go with T-Mobile is because it IS possible to use wifi calling...)

If folks want to experiment with different tweaked speeds, do post the results here :D