you will get more vibration with a single mass flywheel but the upside of that is the clutches available in this type of setup can hold more power, plus the fact that decreased weight in the flywheel (and in turn on the crankshaft) allows for slighter quicker acceleration. the only downside to a lightened flywheel apart from a little vibration is that sometimes you will have to reach for a lower gear as it has less inertia due to the lesser weight and therefore will slow down quicker off throttle. vibration from single mass flywheel i have found is only between around 3000-2500 rpm when decellerating, under load or when accelerating it feels and sound perfect like any other combination, plus you quickly learn to drive around it.
as for power, craig is right
. bottom ends have been known to handle in excess of 400bhp as can the valvetrain (if not spending its life bouncing off the limiter) but the weakest point in the standard engine is the head gasket. if you are changing the gasket then its silly not to change the head bolts while you're there. if you wish to maintain power across the whole rev range then a set of cams definitely helps, and if you change out the cams then a set of valve springs is definitely a worthwhile accompaniment - rocker arm stoppers are just a fairly cheap precautionary measure should a bounce off the limiter be enough to throw a rocker arm, these are meant to prevent the broken arm from bouncing around the head causing all sorts of damage. theres nothing to stop you running enough boost through the same turbo without these additional modifications to make similar power, but i wanted to give enough support to the turbo to make the most of it and avoid any nasty surprises......
