Ive pained for ages about putting a side decal on the Silver mk2 TT, Id say no dont do it unless you do a proper full JDM job on it (lowered springs, aftermarket wheels, body kit). focus on keeping it clean, shiney and original on the outside, spend $$ on good rubber, nice sounding exhaust system, intake, plus interior goodies that make you feel good (OSIR shifters, aluminium bits, leather care, LED light conversion, sounds, custom\premium mats etc.
If you must do a decal, then mabey something lower key like quattro in the 1/4 windows, or some quattro badges on the sills near back wheel.
Silver IMO is actually quite a classy colour for the TT, like the Delorean let the design \ style do the talking not decals and bright colours.