An R8, or any "normal" mid-engined car really isn't a competitor for an SLC, IMO. If all you want is good or very good performance in a mid-engined car, the best value is always in a production car.
I like to think that most people build a car like the SLC because it represents a way to be very different from a production car. For example, you can't buy a production car that is as powerful AND as light as an SLC. Or, for that matter, as light, period.
You can't buy a production car that can be as simple as an SLC (safety, emissions and the plethora of nanny rules make that impossible).
You can't buy a production car that has your choice of drivetrain if you want something different (want a 20B turbo rotary? Too bad, in a production car).
You generally can't get a production car in any color you choose, as you can with an SLC. Or with your choice of brakes. Or Interior. Or suspension tuned your way right from the start.
And most importantly, you have to build an SLC, which means you will truly have a one of a kind vehicle. No SLC is exactly like another as they are all expressions of the individual owners' preferences, budget, and aspirations.
If all of the above is important to you, an SLC can be a great choice. OTOH, if all you want is a good performing car for a low price, the 2020 Corvette beckons.