th TRD parts on the tC dont do much actually, if im not mistaken the only power adder on the TRD tC is th sport muffler, which at best, gets 5whp, which isnt much. but it does have the TRD lowering springs, front strut tie br, and rear roll bar, so it'll have similar or better handlng then the Si. but id say both cars on on par with eachother considering a race between the tC and Si is a drivers race. fastest 1/4 mile time for the tC has been 15 sec even, which is fast considering the power and heavy weight of the tC. and ive seen Si's do similar runs. so it depends on what you really want. tC can be turboed easier with its lower compression ratio, Si has a better supercharger option for it. though at the moment the tC has a larger aftermarket side than the Si now, tC has multiple engine builds, turbo kits, clutch/flywheels, twin disc clutches, electronics.. it goes on for the tC. but both are nice cars and if i coule i would own both, tC for turboing and Si for all motor performance. so its your prefrence. tC has higher reliability right now since the Si has tranmission pop, which messes up your transmission, though this is if you are running it hard, there is a fix for it but it cost money. Scion is rated number 1 reliability and best made car by Comsumer Report and Associated Press.
05 Scion tC owner.