Would it even work?
What I assumed would happen is that the AT&T radio applied to the T-Mobile version would kill off HSPA on AWS.
We know that there have been a few devices over the last few years that swapping radios between the carrier versions enabled other bands.
I also haven't understood what wizardry HTC has done by loading up 2 chunks of aluminum and using them as antennas but we all know it works exceptionally well.
The radio swapping between AT&T/dev/international and T-Mobile is still one of those unanswered questions I have....
sure,it will work fine. the physical radios in m7_ul are all the same,and the bits that enable/disable what bands are able to be used for what(example HSPA on the 1700 band in non-refarmed t mobile areas) are located deeper within the phone than what the RUU can change.
basically,the phone will get all the bands that the carrier intended it to,until its
really figured out how to change it.
examples:
-running att ruu on t mobile device wont break t mo lte or disable 1700 HSPA
-running t mo ruu on att device will not enable HSPA on 1700
-running international ruu on either doesnt change that devices LTE or other bands
personally,unless the radio swap is backed up with speed tests or other real data as above,i tend to not believe any performance claims. i think that more often than not radio swapping has a placebo affect on the user,with little to no real world changes. you also need to swap the ril,as clsa pointed out,and i always go a step further and swap out all firmware. if i want to change radios,i use the hboot,sbl1,2,3,rcdata,etc., that match it. swapping out only the radios on the rezound casued some really flakey/unstable data so i dont do it.
my best advice is "if it aint broke dont fix it"
so if your happy with your phones performance,leave it alone. a firmware swap can brick an s off device unrecoverably in the event of a bad flash,and mismatched firmware can casue anything from failed OTAs,to an unrecoverable brick if a stock recovery tries to apply an OTA
todays phones are complex enuff that swapping things around just doesnt have the same affects it used to. so swapping your radio for international,dev,or other, may give better performance,but could have the opposite affect if not done correctly.