The clutch will slip more easily in a lower gear when there's a lot of torque. At 60 mph in 6th gear you're just lugging the motor and risk other worse damage.
This is what happened with mine. The first signs of slip were in 4th and 2nd, giving it full beans while in the power band.
After that started happening, and figuring a replacement was imminent anyway, I checked what would happen flooring it at low rpm in 6th and it was fine. Slip as described above got worse and worse but 6th was always fine. Weird?
Stock made it about 2 weeks after going stage 2 at 45k miles, with 10k miles of stage 1 before it.