Slightly confused. So the previous owner K04'd it, then put it back to K03 spec but left the K04 map on it, and then sold it as a 'GTI'?
Hmmmm. Anyway...... If you want to keep it as a K03, perhaps consider a K03 hybrid, which will have proper 360 degree thrust bearings in it (stock are 270 or something naff like that), not to mention better turbine and compressor wheels, so will be a lot better than a plain vanilla K03. Situations like this are a good opportunity to improve, upgrade and come out the other side better off, performance and reliability wise. Run it up to the max of 1.5 bar as Dan said regarding the MAP sensor and all will be golden.