I did a reasonably careful measurement using this image:

http://express27.org/photos/2392.jpg

Assuming the boat is flat, A helmsman sitting at the traveler has about 3/4 of the righting moment of a crew hiking reasonably hard amidships. This means the difference in total righting moment between an at weight boat with a 300lb driver is less than 5% different than one with 150lb driver.

-
-   ( 300 * 0.75 ) + 580 = 805
-   ( 250 * 0.75 ) + 630 = 817.5
-   ( 200 * 0.75 ) + 680 = 830
-   ( 150 * 0.75 ) + 730 = 842.5
-

With most drivers in the 150-250 range, I don't think it's worth complicating the rules for a 2-3% difference. I'll also submit this as evidence, that when it comes to winning races there are more important things to worry about:

http://express27.org/photos/640.jpg