Going from 454 Magnums at 365hp,factory 19p Mirages to
525efi's 550hp is a good starting point 23 Mirages?

Forgot to add 5200rpm upper limit for carbed Magnums
and 5400 for the 525's.

I would get a good baseline with the existing props before you jump to a new unkown prop set. Sure, I understand more power may require more prop, but AT LEAST get a baseline with the new motors and old prop. THEN you can make a more educated decision.

I just bought 23 Mirages to start with.19's will be way too small.