it's not so much that you loose compression, as that the air is thinner, and the fuel atomises easier, having less oxygen in the mix. you will loose some power, but you need to jet for it. the fuel that you buy up there will varry on how it atomizes, so check your plugs to see which way to go. generally, you go leaner, so if the plugs are pretty black, do so. if you plugs are white or lighter than chocolate brown, go richer. when it starts to run better at wot, do a proper plug chop to tell you where the jets should be. then adjust your airscrews and, if needed, the needles. the loss of compression will loose you a little power, but it'll stillo run good when jetted right.