yes and no.
yes, you will see a marked improvement immediately with the t-mobile router in use.
no, you could optimize the settings on your non-tmobile router to match the settings and performance of the t-mobile router... and if you have already done this, you wont see a difference except for a logo on your router.
depending on which WRT54 you have... the G, GS, or GL, you could make it even more effective by flashing it with dd-wrt firmware to give you more settings to tweak for better performance. the WRT54GL is the superior router of the bunch for firmware hacking.
my av was not safe for preschoolers.