I flashed the debranded ROM yesterday which removes anything T-mob. That being said, I never actually saw this TMOsync service people mention.
I noticed there is a feature on the t-mobile website for it to backup your contacts etc. which I have disabled. Is it possible you have this enabled?
As for warranty, I guess mine is gone until someone provides the t-mob rom (which can be pulled form a t-mob handset if im not mistaken)
That being said though, from experience with other phone manufacturers, warranties are only voided if the software caused the issue and usually only if its a custom ROM, hardware issues tend to get fixed regardless. Im not sure what sony ericssons view is on flashing another version of their own official rom, particularly now they are allowing bootloaders to be unlocked with statements of "may" void warranty. Might be worth checking out before you do anything.