Even if its only mental, the effect of a light bike, especially if you're used to something heavier can be powerful.
I once did some usual training rides with a mate who was often struggling to match my pace (at the time!).
However on one occasion he'd got hold of a top-end demo bike from the LBS, and when I say 'demo' I mean it was full of boutique parts wayy in excess of the official full XTR spec list- very sneaky if you ask me.
This bike was stunningly light, similar in weight to many good road bikes.
Now he didnt beat me up any hills but there was no sense that he was stuggling either, as I said, even if its all in the mind, that sort of difference is worth paying for if you're racing.