My transom mount speed thingy is pretty useless above 30knots - it pretty well matches the GPS upto that point, and then it drops to say 15knots.
If you do go transom mounting the transducers, make sure they're sturdy enough to take the pounding the rib throws at it. I'm on my 2nd airmar now after the 1st one snapped off on my way out to the smalls last year doing about 30knots (it wasn't that calm). The current transducer is better, but even with the screws as tight as they will go, it still manages to pop up every so often.
Thru hull we priced at around £400 for the tug at work - you could quite easily get a transom transducer and stick it in the bilge and it'll work fine. The only thing that won't work from that is the temperature