AKAIK the compression ratio is different, 4BT is 17.5:1 and 4BTA is 16.5:1. everything else is either the same or can be easily changed, like add a water to air intercooler or turn fuel up a little etc.
The difference in power has been reported to be about an additional 7 h.p. per psi of boost which seems high to me. It's hard to prove since dyno testing does not duplicate real cooling conditions experienced when running it done the road. My personal experience is that it was well worth the effort of adding it. I'm only guessing but it seems to add about 15-30 h.p. averaged over fully loaded and unloaded conditions.
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