It is clear for everybody that PSN is not region locked, but many people including me have seen that online conditions are different whether you play with a game from your region or not.
I’m from Europe, and as far as I can tell :
Soul Calibur IV, whose online is already crappy, is even crappier with an Asia or US of the game
Street Fighter IV Asia has more delay than Street Fighter IV PAL
Tekken 5 : DR Online is “laggier” if you downloaded the US version instead of the European one
So may anyone explain me how it works ? But despite those problems, I have never had any problem for playing anyone from any region, only the online conditions are altered.