I'd think that 300bhp has to be a turbo, and probably with uprated rods/pistons/hardware. It would certainly be interesting to drive, as you've then got a power to weight similar to a Veyron. Possibly a bit excessive, but hey, there's only one way to find out.
Personally I'm going for a Zetec 2.0 on twin carbs and mappable management, and aiming for 160bhp-ish. Hoping to keep the car down around 600kg, so I think that'll be plenty quick enough. If I get bored with that I'll get the head reworked and add some lumpy cams. For me it kinda fits with the ethos of the car. Light, nimble, carrying speed through the corners.