In theory,
any motor will get you to 15 fps...eventually.
The best advice I can offer is to download JVN's design spreadsheets (
newer,
older) and start fiddling with numbers. The newer spreadsheet is a little more user-friendly, though the older one lets you have a little more fun with numbers (particularly when you're trying to figure out how quickly a robot will accelerate).
Another thing worth noting in the spreadsheet is the output at maximum load. Conventional wisdom says that the safe route for a drivetrain entails being able to slip your wheels/treads/doohickey in your lowest gear when pushing against a wall, and do so while drawing less than the 40 amps it takes to trip your robot breakers. If you want a higher speed than what will allow that, you'll either have to add motors or shift. (A good thread on the matter is
here.)
But to answer your original question, it is possible to use four CIM motors with the right gearing and wheel choices and hit 15 feet per second. (Matter of fact, you can even do it using COTS parts exclusively.) I'll leave the math to the reader, since all the tools are already posted.
Hope this helps!