Theoretically, yes: the tire should have 10 to 15% slip for maximum coefficient of friction under dry conditions, that gets a COF of about 0.7 give or take. Sliding (100% lockup and full slip) drops slightly to 0.6 or 0.5 depending on the tire compound. The difficulty is modulating to achieve that slip exactly. Even with a very responsive brake lever and a skilled operator, modulating can quickly go down the curve into the far-too-little-braking category. The COF curves below 10% slip are very steep and immediately drop way below 0.5.

In other words, locking up the brakes do indeed drop off the COF to less-than-maximum--but not much! Whereas letting off the brake just a smidge too much will immediately drop the brake's COF way, way, way below the slightly-reduced COF at full lock-up. So in real-world panic stops on dry clean pavement, there's a much better chance of getting stopped by staying on that brake HARD rather than backing off and trying to modulate.

That's the beauty of a well-designed 4 wheel ABS in a car. The wheel speed sensors and modulating valves can react far more quickly and far more effectively than a human pumping the brakes.

edit: Oh, and when I wrote "staying on that brake HARD" I'm referring to the front brake. That's where the stopping power is.