Im considering the Groub buy FMIC, could someone explain the advantages of this over a smic, other than looking real mean?

Bigger surface area = more cooling = more hp.

how much hp? or does it depend on the setup of the car?

In general terms due to the lower inlet charge temp you'll get more bhp, from hihger boost levels sustained over a longer period i.e. over a greater amount of the rev range. Or something like that.

Amount of bhp will depend on your map.