My memory's foggy on this topic, but I think you'll see about 25% power loss at 7,000ft. Your guess of 50 missing HP is probably on the mark!
As for why the trans shifts harder, that's easy. Because of the reduced engine output you are using much larger throttle openings to generate the kind of acceleration you are accustomed to. The trans shifts more firmly the more the throttle is opened, hence the very firm shifts. No magic there.