VW - Your premise re being able to maintain the same bat speed with a constant weight bat of varying lengths is faulty. Exaggerating to make the point, you clearly wouldn't be able to swing a 10-foot long 20 oz bat as fast as you could swing a 1-foot long 20 oz bat (this is why figure skaters pull their arms in closer to their body to spin faster).
Additionally, not all bats have the weight distributed throughout the bat in the same manner (e.g., end-loaded v. balanced), which affects center or mass and moment of inertia, which affect how the bat feels/swings and, utlimately, swing speed. This is why some bats are offered in multiple drops for the same length (i.e., DeMarini CF3 comes in -8, -9, and -10 drops). And, on top of that, not all 20 oz (or pick any weight) bats actually weight the same, even identical models from the same manufacturer. So....I think you need to settle on the weight/length combination where you can achieve your near maximum swing speed without sacrificing technique. If, like many of us, you don't have access to a bat speed measuring device to quantify swing speed, you can get qualitative results by hitting off a tee and measuring the average hit distance using different bats - of course all the other variables need to be be constant (same balls, temperature, wind, hitter fatigue, etc). IMO, selecting a bat is an individualized trial and error process. But the good news is that, ultimately, performance comes down to the archer, not the arrow! Oh, and if DD doesn't like the grip, feel, name, graphics, color etc., the best bat in the world won't work for her!
Glad to have helped (muddy the waters further) - GM
