Let's ballpark these for ppm of system productivity.
P1 GCD 1 hour at 880M on Xeon Phi 7210. I have another similarexponent P1 that's projecting a week left to go for about half of stage 2. So let's assume 30 days for both stages on 880M, 7210; 60 minutes x 2 stages / (30x24x60) x 15/16 ~2600 ppm = 0.26% of P1 time, which is ~1/40 of PRP time, so ~62. ppm of exponent (TF + P1 + PRP) time. That might become worthwhile to pursue at some point, depending on what other optimization opportunities remain and effort needed.
Preallocate PRP proof space 3 minutes at 500M on Xeon Phi 7210.
Forecast PRP time 328.5 days ~473040 minutes. 3/473040 x 15cores/16cores= 6. ppm of PRP time.
That would need to be a very quick modification to be worth the programming and test time. Seems unlikely.
