Each race is 300 kms, so each race weekend maybe 600 kms. Total engine mileage for 7 weekends is about 4000 kms. By Wazari's rule-of-thumb that means the ICE looses 2-4% power over its life. This ICE degredation probably affects the H as well. So if 800 hp baseline from ICE and H, then probably looking at roughly 15-30 hp change over lifespan of ICE.
My best guess is that F1-level materials and tolerances reduce the degredation more than the F1-level stresses increase it. But still thinking everyone will be wanting new engines at Monza, etc. And of course the whole power unit will probably be more efficient early in its life...