Any guys did any experiment on this?

impossible to say. A well tuned PHP4 environment will still beat the pants of a standard PHP5 install if that is of any help. But the same could be said if the tables were turned.

Personally I would still be holding off on using PHP5 with XOOPS as there are still some things to be updated in XOOPS for full PHP5 compatability.

Ditto on what brash said, plus XOOPS will only perform as good as the server it is hosted on.

I ran some tests some time ago and I've reviewed comparisons of XOOPS to other platforms. Quite frankly, if your theme is designed properly and you're making good use of the caching features built into XOOPS, you can make XOOPS as fast as you want it to be. I've put sites together that load just as fast as static html.

XOOPS isn't perfect, but it is as good or as bad as you make it. Do your homework and you'll be rewarded.