Don't have the latest Mac Pro, got a late 2006 quad with an ATI 1900XT card and it runs my license of (XS)SoftImage 7.0.1 very nicely indeed.
I've always found the OpenGL performance of XSI to be better than that of Maya's.
Also running the same license on my early 2008 15" MBP with a GeForce 8600GT... not as fast obviously.. but still good enough for low to medium sized scene files.
Anyway, I assume it'll scream on a Nahalem Mac Pro with the latest ATI/nVidia card.