Most aftermarket aluminum heads have a pretty beefy deck on them so I would venture to say at the least they are no worse than a set of light weight factory castings and are probably better that the factory iron castings.
Have you retorqued the heads yet? After a pretty good thermal cycle like that, you might want to check the cylinder head torque on a fresh build-up? I've been told that checking torque twice should be the the bare minimum (after warming up and letting cool down) and the more the better (screw ultimate reliability I say, go for 3 times successively and maybe a 4th time at some point down the week
), losing the clamping load and letting go a gasket will warp things in a jiffy possibly leading one to think that the aluminum cylinder heads warped easily.
As for 230 degrees? the good ol' IC engine produces power right on up to 220 degrees of coolant temp after that it starts to have problems (this assuming detonation from other problems doesn't surface first). although that might be the practical limit for an all cast iron engine with a combustion chamber from about 20 years ago (all kidding aside)
Its odd that your cooling system works better without the thermostat installed, typically without the stat installed coolant circulates to quick and does not adequately absorb enough heat (or transfer out) through the cooling system typically resulting in a overheating problem.
Break down your cooling system and lets see whats going on?