Phil (and all)
Actually I have found the opposite with my current 03' Dual Cam models......they seem to be (well according to my numbers they ARE) more efficient with HEAVY arrows. At least compared to my 2001 Pro38 Dual Cam it would seem.
Here' s what I found comparing the 03' Patriot Dually and the 01' Pro38 Dual Cam.

I used the same arrows for each speed test............a 472gr and a 372gr. As you can see they are (drumroll please! 100grs different )

2001 Pro38 Dual Cam:30" /70#
472gr = 289fps
372gr = 326fps

2003 Patriot Dually:30" /70#
472gr = 289fps
372gr = 319fps

I' m not concerned about string load on either bow, because the bottom line is that they both shot the SAME speed with the heavier arrow but one was noticibly slower with the lighter arrow.......which gives a window for comparison.

I' m going to compare them in reverse........vs. the 472gr arrow:
2001 Pro38 Dual Cam gained 1fps for every 2.7grs of arrow weight dropped.

2003 Patriot Dually gained 1fps for every 3.3grs of arrow weight dropped.

Since I' m starting to confuse myself..... I' ll just leave you with the observation that it would appear that the 01' BK cams are more efficient at lighter arrow weights than the current 03' s.
That said.........Only two arrows does not really make a " test" , and the Patriot also has just about a full 1 1/4" brace height advantage.

I wish I still had the Pro38 DC so I could load up a 572ish grain arrow to see if the Patriot Dually gained any advantage in any noticible proportion to what was lost on the light end of the scale.