Jeeze, if I were you id just upgrade to a proper GTI-6 rather then converting. A lot less hassle, and we dont wanna see more D-Turbos taken to pieces and upgraded away from derv haha!
Id keep it simple fella, dont wanna stray too far away from the 306 ideal, its rather nice as it is.
You considered upping the engine at all, if you can get ahold of a 406 IC, its a fairly easy upgrade in all honesty considering, and can push you in the 130-140 level without losing too much economy tbh.
As for a 3.0 conversion, jib that, id rather have an uprated GTI-6 or possible Rallye, they demand more money come resale time, and get more respect at 306 meets as they are held in high regard. Iirc, some guy on the Pug306.net forums is running 230 out of a GTI-6 XU engine. Cant remember whats he done to it, I dont go on there much nowadays.