Need more power! I seriously think the twin engine would be the simplest and would probably be the easiest on the drive train not to mention using all available wheels to put the power to the ground.
The problem with that thinking is this: unlimited budget means better-than-econobox gearboxes. I'd do an inline-6 or v8 that has a compound turbo setup. It can still be light, has a good powerband, and won't lose traction so easily when power doubles over 500rpm.
The problem is I have little need and no use for something that goes >200mph. I'd rather have a lightweight car that can deal with our shitty public roads as well as it can a smooth track, while having enough power to outrun a decent turbo civic. Even then the most common "fast" car out here is a sti or evo with minor bolt-ons. Nobody daily drives anything fast out here.
BTW about those nevada roads - our freshly paved roads aren't even that smooth.