Both WM and Phoenix Mustangs need mods to handle extra power i.e extra firewall CA glue, stab strengthening and former tripling because of the larger fuel tank, so that's a wash for work needed.
I'd say the Phoenix is a tiny bit faster in the straits but the WM makes up for it in a speed holding pylon turn. Push the Phoenix too hard in the turns it'll high speed tip stall or scrub off speed. The Phoenix seems to land nicer and a bit slower. I had/have both planes.