We have slats in a similar sized dinghy. We have a 2.5hp motor, previously 2.2.

Alone in the dinghy I reckon 4 knots, but it is rare that I would use it as such. Loaded much less, the most important thing is to stay dry and I tend to throttle right back and just cruise in and maybe 2 knots or so, once it starts splashing over the bow it stops being fun, especially if you are in good gear or have shopping on board.

I don't think speed is an issue until you get to little ribs where you are sat at a console and get up on the plane, before that the wet / not wet rule comes into play. Just my thoughts.