There are several CT9 options on the market, including some
hybrid aftermarket options. Unless you need that extra power on the track, there shouldn't be any need to go further.
A common mod is to change out or modify your exhaust manifold, as the standard cast iron one doesn't breath especially well and can start restricting if you start cranking up the boost too high. Since modifying the manifold can crack it, many people make up new ones - and by that stage you may as well upgrade the turbo too. I think this is one of the main reasons upgrading the turbo is so popular.