basically the PDA build of the firmware is universal, the phone/modem is the one that controls the radio and frequencies, and the CSC has all the regional build info.
i was running stock eclair 2.1, and upgraded to froyo 2.2 by applying the registry hack, and spoofing TNZ region, which allowed Kies to update my phone to DOJP2.
this was fine until late December, when i noticed Kies stating another update was available. i didnt read the versions carefully enough, and i flashed the phone again which put me into JJ1 or JL1 or something, which was a backwards step into Eclair again.
at this point i gave up with Kies and started trying to find out how to update the phone directly using Odin etc. I found this post by systemtech http://www.geekzone.co.nz/forums.asp?forumid=97&topicid=74830 which detailed his efforts in upgrading the phone. he identified a specfic modem build that works in brazil? that seemed to be very fast and also compatible with 850mhz.
i downloaded the full XXJPY from samfirmware.com, and Odin 1.3. put the phone into download mode, and flashed JPY across... done in 5 mins and no issues at all. i then added just the JPA modem and reflashed the modem across, this worked fine as well. testing the phone saw me running full 2.2.1 with everything working, calls, txt, hsdpa data, wifi, etc.
i then decided to try Darky's rom as prompted by systemtech, this was the brand new v8.0 rom that was released about 4th Jan 2011. clockworkmod recovery was already on my handset as i found out, so all i did was copy Darky's rom into my internal SD memory, and run it from clockworkmod. instantly i had darky's rom working, but it had a weird modem build included. i just reflashed the JPA modem back on, and presto - full darky build, 2.2.1 (dubbed 2.3.1 by him, just a naming thing its not really gingerbread - yet) phone runs lightning fast, and after turning off "wifi always on" the phone lasts easily 48hrs with normal use, and about 30hours with moderate use, with all the added features like upgraded mail app, hacked camera, 5 icon touchwiz launcher, all sorts.
phone is like a brand new person now and there are updates flooding in constantly. so to all telecom users, theres no drama with getting your hardware up to date, just get a good speedy modem to flash on and all should be fine.
just wanted to share my experience for the likes of Shrapnel and defragger and other XT users who might be stuck or frustrated - i recommend the links in this post for alot of what i have described http://www.geekzone.co.nz/forums.asp?forumid=97&topicid=74830