Podds I don't agree with your last point, there is no good reason preventing anyone getting fast. What limits us is ambition and commitment. I'm sure both you and I could easily add 2mph to our Ironman average bike speed if we tried.
Hi Barley - I don't do enough structured cycle training I suspect to say 100% I couldn't get faster but if I look back at my running PBs my mins/mile for my 10K PB is frighteningly similar to the one for my marathon PB and that was despite event specific training including speedwork in each case
I've sort of assumed that cycling would be similar, for me at least, but this may not be the case - I do know that losing the lard would make me faster though 
I guess what I'm trying to say is that if you are a BOPper it is way too easy to put too much pressure on yourself re speed as you are hyper-aware of cut offs and early on in the training season that can be really off putting and hit your motivation
For some people (me and I suspect EP) there is a need to have a realistic idea of your speed and what you need to make cut offs but without aiming to just duck inside them which I agree is not a good target to have per se