The majority of the FAA instructors at GYR will not sign students out solo above the Piper limit of 40deg. Some will sign students out up to the OAT limit of 43deg, and some will fly dual above this.
According to OAT, who have talked to a source at Piper, Piper recognise the use of interpolation using the density altitude to determine the performance figures, but not extrapolation of the current charts. As yet there is nothing in writing from either Piper or the FAA, but OAT say they are expecting something soon. However, looking at the other posts here, I am expecing to be waiting a while!