robfish:I haven't had the original telecom modem since I flashed new Rom a week after I got it and have no issues. Mind you I'm on custom Rom but don't see issue if you flash one of the stock current ones.
Don't you need to make sure you have the correct frequency modem (850MHz WCDMA for Telecom I think)?
Is there an official ROM available from SamFirmware which will work well with Telecom XT network?
All modems work well. Haven't noticed any major difference. I flash most custom roms and just go with whatever the developer installs. Most of the developers include a few modems with their custom roms but usually install stock (as whatever's in the latest stock firmware) as default.
Currently on DXELK1 on CrDroid v13 based on XXEMA2 firmware.