The L/D curve can be considered a sum of many individual effects. The effect that is predominant at low speed is induced drag. It increases as the speed squared decreases.
The effect that is predominant for helicopters at higher speeds is profile drag. It increases as the speed increases.
If you create a machine that is designed to operate in the low speed environment and to carry a cargo volume, then it will have a relatively high cross sectional area, leading to relatively high profile drag. This will tend to shift the max L/D point towards lower airspeeds.
A good test of this (which may just prove me wrong) is to look at helicopters that aren't designed for cargo volume such as Comanche or a clean AH-1W. Does anyone have related numbers for these machines?
Matthew.