a race is 80% rider, 20% setup... and of the 20% setup its 75% suspension and gearing and 25% motor...
theres more to winning a race then slapping big parts in a motor
id stick with whats been proven time and time again.. dont fix whats not broke.
Edit: also a 4 mil produces about 7% more power ovr stock. so the extra 1mm of stroke your looking at half a percent... whats the price of this 5mil?
also a side note to make sure we are all on the same page... your talking about a 5mil stroker and not a 5mil long rod correct? those are two differant things.