I've been following this formerly informative thread for a little while now, and it certainly has taken a turn. Holset seems like another alternative and this thread has peak'd my interest in them. I'm geussing the real reason you don't see many holset turbo's is the lack of good information.
For those that have researched turbos, you would know that what matters in a turbo is not the application the turbo came from, but does the compressor fit your needs, and can you find a suitable turbine housing for your application. I'm far from any kind of expert, but those seemed to be the important things I found after a little research. Holset seems to be a match in these areas.
Yes they are diesel turbos and as such are designed to run higher pressure ratios but from the little bit I have found with my limited knowledge, they seem like a good fit if you plan on 15+ lbs of boost.
Here are some HX/HY-35 maps. Boost pressure at 20 psi (pressure ratio calculated from that), with cfm plotted from 500 to 8000 rpm. Pre-2003 HX-35 in Green. 2003 - 2004.5 HY-35 in Black. 2004.5 - Present HY-35 in Red. This is for 2.0L of displacment. (Excuse the updated graph, as I have some small changes yet to make to the fonts and scales, but the maps are correct.)
http://www.dsmtuners.com/forums/showpost.php?p=1201146&postcount=9
The newer hx40 with the "Map Width Enhancement" housing sounds like a great turbo but I can't find any maps, LINKS anyone?
These "diesel" turbos seem to be on a variety of applications from 1.6 litre hondas, 2.0L nissans and DSMs, svo stangs, audis, volvos and 450+ ci big blocks.
Here is a holset powered svo mustang
http://www.youtube.com/watch?v=bT7-E-EzTFw&mode=related&search=
And the only supra relavent video I could find an hx40 powered 7m
http://www.amworks.com/video/supra01.mpg
http://www.amworks.com/video/supra02.mpg