A RIB with a decent powered outboard that can do 15 knots or more has nothing to fear from the falls. There is a 12m deep marked channel on the south span of the bridge which doesn't run at much past 10knots even on low springs and is clear of obstructions even if the engine failed.
We do donuts in it at any state of the tide for a bit of fun when over diving the SS Breda, going back and forth through the channel playing around and around. Just stay on the correct side of the channel as if you go through the north side you may hit the rocks on the east side of the bridge at low water. Technically you can drive over most of these at HW but I wouldn't recommend it.
Coming back down is no issue either in a decent RIB of the size quoted, just much faster than going up at LW springs