I put the cost of tires, engine, fuel, brake pads, and gearbox at $351 per hour for Spec Boxster, and $263 per hour for SRF3. So about 25% less to run SRF3.
Operational costs of racing classes
And of course, if you have an incident, you'll spend a LOT more to fix the Boxster (as you know!).
Lap times are about 4 seconds per lap faster in SRF3 for laps around 2:00.
On the other hand, you can't drive the SRF3 to the track!
Greg