First, Chop off the muffler and replace it with a cherry bomb glass pack.
Upgrade the intercooler to one of these.
TyrolSport UG SMIC Intercooler For VW MK4 Golf/GTI/Jetta TDI
They not only cool better but they also have less boost drop. More air will compliment your larger injectors and increase the percentage of fuel burned. The reduced boost drop will increase the effectiveness of the turbos energy recovery from the exhaust below maximum boost. It will also reduce back pressure on the engine at maximum boost.