Man, would I like to get a copy of that!!! I started saving some $$$$$ for such purpose, but it will take me a while, lol .
There'ss even an ATI bundle that comes with a FireGL card plus latest version of foundation (4.2) for $599 !!!!!!
link:
http://shop.ati.com/product.asp?sku=2603769
man!!!!! one of the best 3d packages with a workstation videocard for that price!!!! and as Vermeer said, top-notch plugins included (Latest version of MentalRay multi-thread as well).
edit: you'd still need a plugin written with XSI's sdk to get your geometry+lightmaps (GI) + a loader class for Irrlicht; but if Zdimitor did it for 3dsmax I see no reason not to do it for XSI.