You dont need trim tabs on a 5.8m RIB no matter how choppy the saarf atlantic is. Trim tabs are no substitute for a well balanced RIB and are really only needed on larger (say 7.5m upwards) RIBS and larger powerboats. The can be used to aid getting on the plane (particularly say for larger powerboats) and to counteract the 'leaning into the wind' effect that planing boats can suffer from.
For the original poster. You sound as though you know boats pretty well and of course have an excellent RIB to play in. Explore and extend your comfort zone yourself, remembering that all ribster talk in the pub about doing 40kts through Force 12 seas and pah it was nothing. Reality is often very different!
The Scorp is a capable hull and often more soft riding at 30kts than 20. Over confidence can lead to stuffing, impacted spine, ejection of passengers etc etc just as on any other RIB!
Out of the fog......