The 140 treadwear would do nothing to stop R-compounds. The only reason that Hoosier and Kumho haven't gone after the ST market yet is that there isn't enough money in it yet. However, the current crop of ST tires are just as expensive and wear out just as fast as Rs since they have to be shaved.

Here is a read about the history of R-comps in SCCA:
http://www.wincom.net/trog/street_tire_faq/