Sorry I haven't given a pirep sooner,...
Got my bird back in May with my new I0-550 and 400 lb upgross kit. What a difference. To be honest, I didn't expect to get the improvement that I have seen. My takeoff roll has been reduced significantly - my guess is about a quarter to a third reduced. When at full gross (1200 lbs) on floats, it has no issue getting off the water - even in hot weather. The prop (hartzell 8468) is obviously matched pretty good to the engine.
My cruise speed seems to be about 2-3 knots faster at the same fuel burn as I was with my O-520 - about 13 gph. I'm running lean of peak there and cruising around 118 knots. When in cruise, even on a hot day, I can easily run with the cowl flaps closed and around 375 F on my hottest cylinder.
All in all, I'm very happy with this mod and I can haul a huge load and remain legal. Next project is the interior.