You sure you don't have a draining plug Daniel?
My last rib a Valiant V490 (just slightly longer than yours but with the same standard spec) was SOLAS approved. It had a draining plug(actually more like a screw than a plug) just at the very bottom of the transom. I'll post a pic but the quality won't be very good as it a cropped image.
Anyway, before somebody didn't close the anchor hatch properly and the boat took in a fair amount of water as the anchor locker was not sealed from the rest of the hull (major design flaw!!). Top speed was reduced by 3-4 kts and it sat noticably lower in the water and took longer and more power to plane. Drained the boat and she was back on form again. If you definitely don't have one I suggest you have one fitted or keep your anchor locker closed!!