False. The larger rotor is dissipating heater faster than a smaller one. All the caliper sees is a slightly cooler rotor at every point in time. There aren't any diminishing returns to going to a larger rotor as far as power/heat dissipation. Bigger is always better in that regard.
I'm running the 225mm hope up front. I'll probably put the 220 SRAM on the rear when I get around to it. I'm 215 pounds and ride super technical, steep DH stuff with it (30-40% grades as measured by Strava). 225mm rotor is a game changer. Definitely a must do if you are over 180 pounds...
Ya I wonder if it's worth trying to make the bike more DH-y. The front end stiffness would be nice for the high speed rocks. I guess it depends on what you're riding it on. Might be better to keep the single crown for jumping focused stuff.
How's the Öhlins shock? Worth the extra money to...