The best comparison I can come up with is this (not very good but hopefully you get the picture):

Take someone who has just passed their driving test 2 months ago (say a 17 year old that has only ever driven a 1.3 KA - not driven ANY other car up to now)......

Put them in a Standard Cossie (220 BHP) and tell them to take it for a 10 minute drive, he would prob come back shaking, saying WOW their just sooo quick who could want more performance? Then put than into a Stage 1 Cossie (270 BHP) and IMO they would not notice the difference in the performance of the two cars......

But if you took to the owner of the Standard Cossie (who for arguements sake has owned and drivin it for 4 years/40k miles) and converted his car to a good Stage 1 conversion they would notice the difference immediately, the first time they press the throttle in 1st/2nd gear...............and even though the owner of the cossie might say "WOW thats plenty of power"..............within a few months they would have got very used to it..........Chances are (money permitting) they could easily justify a Stage 3 conversion, then Stage 4 and so on.........

The point being if you said to the man in the street (mr average driver with a 1.6 L Mondeo) that you really want Rod Tarry's Sierra engine (if it was for sale) and that your pay £ 25 K for it because the 500 BHP that you have is not enough - he would think that you were mad (pun not intended) LOL