if only you could 'buy' little packets of confidence.
i really think it hard to quantify. if you think you have rubbish tyres and change them to some you saw a guy go round the outside of you on, then you will probably go quicker. but the same would be said if the tyres were originally the other way round!
for me tyres are THE most important thing for your confidence, after all they are only only thing keeping you in contact with the road. if you think your tyres are going off or you could get better ones. changes them.