Hi and welcome,
I have a GT4, (see below), with the 300PS engine. I test drove an 05 car before getting mine and I didn't notice any difference. I did only have the car for a few hours though and then had to wait a couple of months for mine.
One guy I spoke to with a GT4 had an 03 for a couple of years and he says the difference is noticeable. He described they new engine as more willing to rev and a little more low down grunt.
Overall I don't believe the difference is significant though.
Good luck with your search.