XSI 8v (95) - What to do?

[mackat]

I'm about to replace the heater matrix, so while I'm working on the car was wondering what I could do to the engine at the same time.

Please no 'buy a GTI6' comments :p

Would the inlet manifold from a 6 would fit the XU10J2 engine?

What else could I do to squeeze more power out the old thing without costing the earth?