I went for a trip on the F&C "pub taxi" and had a long chat with one of their members who took (and wil lbe taking) their "model puffer" (Maryhill) up. I took copious notes which I will need to translate into "rib compatible" notes, as their boat only does about 6 knots flat out, and draws a bit more than your average RIB, so the "leave here by X o'clock to ensure enough water at Random Mudbank whilst steaming at 6 knots into a 5 knot water flow" notes will need modified!
There is a seriously rocky & shallow bit round the corner form the Rowing Club (I saw photos) that is only passable at high-ish water, and there is the ford at Cambus that causes all sorts of tidal weirdness upstream, although wit ha rib's power - weight ratio it shouldn't cause any problems.
I'll post back once I have figured it all out, but those times seem to be reasonable.
SPR, you got mail!