I too was looking into Turbo'ing my VR a couple of years ago and spoke to Vince at Stealth in great length about doing so.
Long story short I ended up Supercharging it instead, all I can say is Vince stated that standard internals are good for 400 bhp, mine is standard and is running a modest (by todays standards) 290 bhp.
As Noggin says above, ring and speak to Stealth as they'll be happy to point/guide you in the right direction.