IIRC when RIB International Magazine tested a smaller Selva a few years ago they had some serious concerns about its handling.
Check out test report on http://www.ribmagazine.com/index.htm
Selva outboards are old tech 2 stroke - thirsty & noisy. Not at all popular in UK but cant comment re. Spain.
For the rig you are talking about you perfectly illustrate the nonsense of RCD classifications. 14 persons in a 5.6m RIB? I'd rather not thanks!
I reckon you would get top speed c30-35kts depending how many peeps up.