I'm not one to try to guess between a hybrid and a roadie, but comparitively I'd think you might see about the same amount or more between a MTB and a roadie. Same reasons, except that a MTB usually has even lower gearing, and those knobbie tires offer even more rolling resistance. Throw some street tires on and you'll see some improvement and will be able to lean toward the taller gears. For what it's worth looking back, I saw maybe a 1 MPH increase in avg. speed from MTB to hybrid, and about 2 MPH increase going to roadie.
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Tom
"It hurts so good..."