I wouldnt, but thats me. If its a 4 bar pump, then it isnt big enough to start with anyway..
The GTA turbo runs 3.2bar with atmospheric pressure to the fuel regulator. When you run a bar of turbo boost pressure, fuel pressure is raised accordingly by the same amount, so on boost, you are running 4.2bar fuel pressure. Hence why the original pump is 5bar..
Managed to order the correct Bosch pump from a motor factors.
For reference in case it's of use to anyone else, I've ordered it from CES in Chester, Tel 01244379531, £91.70 +VAT and the Bosch part no. is 0580 464 027