I posted my experience comparing the 7in, custom 7in HD and ER competition FMICs here
VRSF 7in vs 7in HD comparison
www.e90post.com
Tentative conclusion: Even if you are only running 400hp-450hp, you still need the largest FMIC you can get if you are ever thinking about hitting the track. On the street, any upgraded FMIC is probably OK.
My cars FMIC history goes like this: Stock - 7.5HD - 7 nonHD - 7HD "custom" - ER competition w/ water injection and water spray. The benefit of the ER fmic is that has the same frontal area as the 7.5HD / large wagner but without being stepped, so you can extend the coolant radiator down behind it. BUT I have yet to verify if this is actually beneficial to water temps or not. I have mounted a ball valve so I can turn the radiator behind the ER on and off and do A-B testing next time I hit the track this month.
When I went from 7.5HD to 7nonHD my tuner reduced timing due to excessive iats on track. I ran that setup because I wanted to test having an additional oem sized radiator in front, effectively doubling the thickness of the main radiator. I no longer run that setup, as I am now back to what is essentially a very 1M coupe-inspired configuration.