@thor In my experience, MBP will throttle anyway under continuous high workload, it's only a matter of whether it happens in 2 minutes or say 20 minutes, but they are just not designed to run under 80% load for hours — that's what Mac Pro is for. Laptops are perfect for stuff like processing photos with short bursts of CPU load.
I don't think changing the thermal paste will help greatly, just cleaning it would be enough.