[Galaxy S6 General] djshotty:
Okay! Here's the lowdown and full explanation about running 'F' variant stock firmware on an 'I'. Please read this carefully before hammering me with questions, as I'm in the same kind of rowboat as you, only a little further downstream...

As far as my understanding of the hardwares in both the 920/925F and 920/925I devices, they are indeed identical. Only difference between both variants is the firmware Samsung have released to operate the devices in the regions of the world they were sold in, and how the System-on-a-Chip (SoC)'s LTE processor on each device can connect to different mobile service provider network bandwidths (on GPRS, EDGE, UMTS, HSDPA, HSUPA, HSPA+, LTE, LTE-A). According to the spec. sheets on pdadb.net, there is only a small discrepancy with what mobile bandwidths each device can connect to which the other cannot, as follows:

920/925(F) - can connect to LTE800 (B20)

920/925(I) - can connect to UMTS1700/2100 (B4), LTE700 (B28)

BOTH devices are capable of communicating on the following bandwidths:

GSM850, GSM900, GSM1800, GSM1900, UMTS850 (B5), UMTS900 (B8), UMTS1900 (B2), UMTS2100 (B1), LTE2100 (B1), LTE700 (B17), LTE850 (B5), LTE1700/2100 (B4), LTE1800 (B3), LTE2600 (B7), LTE1900 (B2), LTE800 (B18), LTE900 (B8), LTE700 (B12), LTE800 (B26), LTE800 (B19)

These differences lead me to surmise that the stock OEM firmware for each device has different modem.bin files, ie: the files which tell the LTE chip how to communicate on certain bandwidths, and Radio Interface Layer library files. Why would Samsung build two devices that are identical in every way, except for a very small section of the very same Exynos 7420 SoC? This leads me to believe that all a 920/925I user need do to run a stock 920/925F ROM is flash in a 920/925I modem.bin once they're up and running, and the 'radio' libraries will be updated for your 920/925I to work on the UMTS1700/2100 (B4) and LTE700 (B28) bandwidths. Only way you'll know for sure is to try that solution for yourself; a solution that's actually worked for me on the three occasions I have flashed a 920F custom ROM (one cooked using stock 920F firmware) into my 920I, and connecting to Telstra's 4GX network at speeds approaching 250 megabits per second downstream. Everything about each stock firmware released by Samsung is pretty much the same, except for region specific bloatware, Country Sales Codes that control how the device connects to the individual Mobile Service Providers in it's home region, and region-specific libraries for driving the LTE chip.

Only thing to be careful of? You run a 920 ROM on a 925, or vice versa, AT YOUR OWN RISK!! You may have heard of custom ROMs that include features specific to the Edge, and I myself am running one such ROM, but I have never grabbed a totally stock 925 ROM and flashed it into my 920I.

If you would like to see what bandwidths your exact model variant SM-G920(?) runs on out-of-the-box, then visit SamMobile's thread on the subject here:

http://www.sammobile.com/forum/showthread.php?t=26440

In closing, I again post the list of the 68 Country Sales Codes for regions where both the 920I and 925I was released. I have of course checked every one of them using the latest SamFirm tool, with nothing showing up as having a Marshmallow 6.0.1 stock ROM available for download from Samsung's servers. Sorry to disappoint you...

ARO BAT BNG BVO CGU CHE CHO CHL CHT CHV CHX COB COD COE COM COO CRC CRM CTI CTP CTU CWW DOR EBE ECO EON ICE INS INU IUS JDI MM1 MNX NBS NPL NZC OPS PCT PCW PET PGU PNG PNT PSN PSP PVT SAM SLK TCE TEL TGP TGU TMM TNZ TPA TTT UFU UFN UPO VAU VFJ VNZ XSA ZVV ZTR ZTA ZTM ZTO

---------- Post added at 13:19 ---------- Previous post was at 13:08 ----------

Have you tried to yourself, and failed, or have you solid proof from other 920I owners that flashing the stock 920F on a 920I won't work? Let me just say this: I am 75% certain any variant 925 stock ROM won't run on a 920, and vice versa, but after doing a bit of research and testing of my own I can tell you I've had a 920F custom ROM working beautifully on my 920I. This came about because I Odin-flashed the modem.bin from the newest stock Telstra Australia 5.1.1 ROM, immediately after booting up a TWRP Recovery flashed custom ROM which had been built exclusively from the newest 920F stock ROM. That same install is still to this day my daily driver, and has been since Christmas Eve 2015, so I reckon you've made a boo-boo somewhere along the line if flashing a F release MM on your 920I has indeed failed...