Hello mrx,
The official info:
http://www.rbproducts.com/cgi-bin/ub.../t/002672.html
Scott Kimbrow has run both the Xenon and Xenon R (pre-production versions). The "R" is a road rocket. Good milage and loads of power. For release info watch the RB site. The Xenon's have been discussed in the "RB Engine thread" listed below. I hope this has helped.
Rich