Just to clear things up, the block guard doesn't completely eliminate the water jacket. It's like a cap that goes over it. Flow of coolant into and out of the jacket is restricted a little bit but I wouldn't be worried too much about overheating as long as there are still some coolant passages.

My thoughts on the matter are that if you're making enough power to create a need to reinforce your block, you've got enough money that you might as well have new sleeves installed.