I don't think MkV would be much faster (if at all) than MkIV fit with the same turbo and outputing same power.
It's not just the grill, the twin middle pipes I do not like either. And you cannot really put MkIV front on MKV. MkIV just looks more purposeful from the front, actually I like the whole exterior shape of MkIV more than MkV.
I don't know about interior being dated. The dials are nicer on MkV Golf, definitely. But it is also common knowledge that MkIV Golfs were built and detailed really well, better than MkV.
MkIV R32 DSG single turbo: 400hp/515Nm = 12800 euro
MkV R32 DSG single turbo: 405hp/530Nm = 16000 euro
