Good day to all! I don't even know where to begin. Am new to FI engines and am residing in Malaysia. Just took into possession 2 months ago of a 2015 WRX STI (JDM) and since the modding needs are strong within me, would like to seek some advise on my direction. It's a weekend car with about half a dozen trips to the Sepang International Race Circuit yearly.

Have recently changed the following and it made a great deal of difference in shifting.
Perrin Shifter Bushing Kit
Perrin Rear Shifter Bushing
Perrin Transmission Mount

My next move would be to replace the rotors, though a 6-pot conversion is on the platter as well. Overkill? Perhaps, but I love the bite it produces when I had it on my JDM Civic FD2R. Not that the current Brembos are bad.

Another option is a direct replacement on the cast pistons to a forged piece. My consideration is on the Cosworth model.
Cosworth Forged Piston Set 99.5mm 8.2:1 CR Subaru Turbo 2.5L(inc. 2006-2014 WRX / 2004-2015 STI) 20003674 at

Thus, considering that I may want to opt for a similar size pistons of 99.5mm bore, would I require a retune if I should opt for this route? I know it may sound silly that I would opt for the same size since I am already dissecting the engine. My main concern would be on long term reliability, not so much on gains though that would be welcomed in the near future, hence the forged option for stability. The lack of reputable tuners here is a headache for me, hence my concern with changing the size of the pistons. Am I right that the stock conrods are forged?

There is such a tonne of parts that's available, and unless I have a bottomless pit of moolah, I can only and slowly, mod what I think my direction would fit me best. Appreciate any suggestions or advise especially so on the internals.