why? because you mention you want power.
i would suggest you to go for changing the BOV first, the exhaust manifold aka extractor is around 1.2krm custom made, theres no pre-made.
something like you can find in wei yip or wai heng exhaust shop
why i suggest you to go for BOV, is because in the near future when you wanna boost up higher then your stock boosting, with the stock bov thats come with ya gsr, it might kill your turbine, eventually, the stock 1 are good for stock boosting if i'm not mistaken is at 0.65 bar. so when you boost higher that time, the bov will not be sufficient enough to vent out all air to atmospheric, and result a return compressed to your compressor wheel.
btw, stock gsr boosting is at 0.65, it will spike to 0.8 and then drop back to 0.65.