Does this extra instruction of the i3/i5/i7 have usefulness for PCSX2? Strange enough I can use SSE2 to SSE4.1 with GSDX and see a 0 FPS change in all games.
That's because you need to test it in scenes where it matters. Those are rare, and you need to have a little luck to find them.
Also it needs very precise benchmarking. A quick check if that intro is 60fps or not won't cut it.
SSE 4.2 doesn't seem to bring any good new opcodes, at least nothing I can think of.
sse4.2 is useless for pcsx2.
AMD's sse4a's EXTRQ/INSERTQ instructions might be fun for something, but it really won't make much of a difference speedwise.