the larger turbo with the other supporting stuff does offer a small increase in power but is no where near as punchy it's more linear doesn't come on almost instantly like the td04 does by this i mean td04 snaps into boost td05 builds it steadily.
there are front housing and compressor wheels available on ebay stick to trusted ebay stores like kinugawa. i beleive the gsr is a 13T
so something like this
http://www.ebay.com.au/itm/Turbo-Compressor-kit-Upgrade-SUBARU-TD04L-13T-to-19T-/271355205508?pt=AU_Car_Parts_Accessories&hash=item3f2e07d784
would see your turbo flowing alil more reducing top end taper (boost falling away)
