I am pretty sure they were referring to the graphics cards available on Linux, many of which are the top end ones on the Windows platform. In addition more current generations of Intel boxes are faster than the older IRIX based Octane's and O2's they had been using. I don't think this is a reflection of Linux being alot faster than IRIX (both being UNIX) just that more current Wintel hardware gave them a boost.
They couldn't use a Windows based system, since most of their pipeline is on IRIX (UNIX), but when Softimage and some of the better graphics vendors made a Linux port, they could now take adantage of the cheaper hardware.
FYI: Windows wasn't an option because of the closed nature of the platform. Studios that do a lot of R&D and can develope their own tools need to tweak the OS for their own needs. Windows doesn't allow this except in very rare cases. However, Linux is obvioulsy useful for this since it is totally open, so most studios on IRIX are planning on adopting Linux as IRIX runs it course. IRIX will still be around for along time, but SGI is focusing more on government and the very top of the graphics market for VR work.
Urp