Okay, since you posted this question in the Racer's Forum, I can tell you right now what the consensus will be: every tenth counts, most every change changes your parameters.
But from a street perspective, your original statement carries some validity. Select a convertor based on your engine build; ie intended use, torque curve and RPM range. If, after your build has been sorted out and finalized (and happy with it), you feel you could benefit from a custom convertor (and you usually can), then have one built to maximize your final setup. Otherwise you get caught up on the tweak this/change that merry-go-round. Most "street only" guys would like to arrive at a final, satisfying combo that they can drive and enjoy, and not worry about constant wrenching to wring out that last hundredth.
Been on both sides of the fence, so I'm just being Devil's Advocate here and adding my 2 cents.