I've had both versions of Audyssey you are asking about. My short answer is - no. I owned an Onkyo SC-885 for some time then sold it and bought an Integra DHC 40.1. Both are pre/pros and do pretty much the same thing - the 885 had the XT version and the 40.1 I have now has the Mult EQ version. I suppose if you have a very difficult room to correct for sound reflections and peaks and valleys the XT might be 'night and day' different. In my home it wasn't.
As always YMMV.
Audyssey.com explains the different versions:
Flavors of MultEQ
Audyssey room correction technology comes in four solutions: MultEQ XT32, MultEQ XT, MultEQ and 2EQ. While they are all built on the same core science, each is designed to operate within the constraints of the available DSP processing power.
MultEQ XT32
Our newest and most accurate room correction solution with more than ten thousand individual control points allowing finer details of the room’s problems to be captured and corrected. The ultra high resolution filters are applied to all channels including the subwoofers, with the most obvious benefit being heard in the low frequency range where correction is needed the most.
MultEQ XT
Our advanced resolution room correction solution with high resolution equalization filters for satellites and subwoofers. Most products with MultEQ XT are
installer-ready and can be calibrated by an Audyssey Registered Installer to provide even higher performance for even the most demanding large or odd-shaped rooms.
MultEQ
Our standard resolution room correction solution that uses mid-level resolution filters for satellites and subwoofers.
2EQ
Our basic resolution room correction solution that uses basic resolution filters for the satellites, but does not apply a filter to the subwoofers.