yeah, a stiffer board will help with speed because your stability will be greater.
Since you're looking at an all mountain killer, you might want to look into the NS SL. The NS vario grip works the same as magnetraction and with the C2 BTX, Mervin took that technology from NS.
If you're dead set on mervin, then the TRS BTX would be a good choice too.
As far as bindings, the 390's are a bit too soft for all mountain so get the targas