They'll definitely help on a turbo application. You generally want to decrease overlap between the intake and exhaust events on a turbocharged engine so you don't bleed as much boost away as you would with the stock setting, so yes- they can help quite a bit under boost, even with stock cams.
$400 isn't too steep for some makes of cam gears. Some of them are easily that much. I only paid $185 for my Skunk2s brand new though... look around for some sales or group buys, and you should be able to get a set of the same for much less.