I'm not really sure, if I have a lower planning speed unloaded. If I do it's not something that i have been thinking about. So it's not much.
maybe 1-5 knots I'm not sure.
But I have noticed a big difference when powering up the boat with 6 divers in it. It's a lot faster to go into planning speed. Normaly I would have to move the divers up front. I don't have to do this anymore.
Rene