The problem with a flexi tank would be splitting it - imagine filling it up at 60mph ......... the rule of thumb is to have at least as big breather if not double as the inlet. We use a big bilge pump to empty ours, but is has a well designed sump system to make it work.
You will need to empty and fill reasonably fast or it will be full when going downwind - not the best situation!
As far as comments like "RIBs are a bit light in the bow" - you should know by now (it must be on Google somewhere
) that not all RIBs are the same! Prop choice, rigging, driving techniques and 101 other factors all have a part to play!
Never heard of any racers that use them?
Another use if you're going to plan on doing very long passages is a forward fuel tank, but this would mean modifying the boat and fitting an extra forward tank and a tap system.