i myself tried RT215 many years back and not 615. also used federal RS but not RSR. RS is light-years ahead of RT215 in terms of wet grip but that does not answer ur question hehehe
funny, i got the same experience.
use the Rt-215 a few years back at Streets of Willow springs doing 30 minute sessions with other cars and great survivability and consistency for a total track seat time of 2 hours 30 minutes.
currently using the Federal RS for Hillclimb, Circuit and slalom here in the Philippines now for 2 years, durable tire with great grip, just takes a little longer to get to great operating grip temp at the circuit track, great treadwear at 180.
looking at the RS-R's for replacement, but would definitely want to try out the FZ's
Tevs