**Heres some of my reasoning on some points already raised....**
C-40 said:

That table only applies to one hypothetical person. The measuremets required to calculate "optimal crank length" are very difficult to take with enough accuracy to produce a valid result.

I agree with you there...but I was trying to make the question easy by using that hypothetical rider to illustrate a point, Like a lab test..ie cut the variables and measure one thing the crank length. I never took these measurements from myself as you say too DAMN HARD to get acurately, so I did the next best thing just looked at their base case table on that page, and saw the watts at 172.5mm -> 543 watts at 175mm 555 and worket out the rest from there...then I said to myself " If I was that rider what would a 2% increase mean to me when I used my power values in place and the answer was 4.8watts rounded to 5 watts!

C-40 said:

If it was true that longer cranks always produce more power, then why aren't all the pros using 180mm or longer? Surely they all want the most power.

....Longer cranks within the limits of their range of motion for that specific rider, so obviously a really short guy wont have huge cranks cause he'd be outside his limits of whats right for him so...

....also I know Lance uses 175's for road and 172.5's for Time Trial stages, he's 2 inches shorter than I am and he's on 175's?? true I dont know his leg length but I wouldn't imagine it'd be proportionatly longer than mine given Im 2 inches shorter than him and my legs are not exactly short....

Turbo Turtle said:

Where does this 5 watts come from? Do you think it's magic? The power comes from your legs and you only have so much. If you are going to turn a bigger circle with the longer crank, then you have to turn it slower. Power to the road is the same

...yeah turn it slower for the SAME POWER!!! .....well if you turn it the same speed (cadence) the leverage is greater with the longer crank so therefore the potential for more power is available ie the 5 watts....more power for same effort at same cadence is the theory...

You have looked at the link ye? nothing here will make much sense without that!

and Thanks for everyones input so far!