Given that the 5W-50 starts with a basestock lighter than a 10W-40, I can see why they want the 15W-50 for high performance, because it at least starts as a 15W (*)
And the technically correct way for them to proceed with this engine is to cut the power output as the 5W-30 is MORE than adequate, in fact it's generally too thick (*)
(*) these are all things that I've learned here...
actually 15W-50 makes a fair bit of sense on the track.