1.8t upgrade routes

What's the usual upgrade route on a 1.8t, custom manifold, cat and back box. 

I hold my hands up, I have little knowledge of turbo engines. If you were to upgrade from the K03 turbo to the K04 is the fitting the same on both turbos? Remap ecu is obvious but what engine upgrades are needed to take the new turbo? I have heard the k03s drops delivery towards the top of the Rev range so scrubbed that idea. Is this correct? 

If there's a usual route people go down would you let me know? 

Thanks