Yes, it was strange to let destroyers travel faster than an plane... but then this is a game and common sense does not really apply.
I changed the speed to be 3 as you suggested and modified the turn count to 8 so the total range was still 24, played a half dozen games (vs 6 varied computer opponents) with these settings, and found that it make for more interesting game play (at least for my style of play). It better balances the trade-offs and utility of the air transport against the utility of a troop/ship transport... but I think the cost to produce might want to be increased very slightly... or not. Hard to say as I haven't tried any games with an increased production cost yet, but the idea of changing the speed seems to be a good one.
Gary