Which components actually make a difference?

I tried to be quite specific and I said that durability wasn't a consideration (don't fight the hypo!). Everything is a compromise as you say.

I'll put it another way. If you did a properly double-blind test with two identical bikes except for changing one component, changing which components would make you feel an immediate, **substantial **difference in comfort or speed?

I think a blind test would cause you to crash before you noticed anything.

For me the answer would be saddle. I would not notice changes in anything other than tyre pressure. Oh and position.

Clipless vs. flat pedals is a pretty obvious difference..