I'm not sure if the aftermarket unit does it but I know on most modern cars the AC compressor turns off when you rev the engine up really high so you won't have any power loss. You just need to remember to turn off the AC for hills and such which wouldn't be that big a deal. I'd rather be cool and driving slow then flying up the hill and be sweating like a pig.
I have AC on mine, and the power loss is not THAT bad. In fact, when I drive from Panama to San Jose, Costa Rica, I always do it with the AC on, and that drive, apart from being 520 miles, goes from sea level to about 3800 feet up (and considering San Jose is in a Valley, and that you have to go OVER the mountains to get here, I probably go a bit higher than that)...