A good friend of mine has been running 9.70's in a street/strip Fox Mustang for three years and no issues with one of these guys trannys http://www.competitiontransmission.com/
Here I have been waiting for someone here to mention Paul... but he really doesnt do transmissions anymore does he..
Depends on what the customer wants. A "full-tilt-boogy" c4 is very expensive, but any transmission to handle 1000+ hp is expensive.
JPT was the original "innovator" of the c4 but he has recently retired, I have his cell phone # and stay in contact regularly. Alos, Pro-Trans in Lancaster CA is an excellent company for custom C4 parts and one of the current leading innovators on the c4. I am only speaking of Hardcore c4 parts, the generic stuff can be had anywhere.