I agree with
@R8-Stuff regarding the difficulty of properly designing an exhaust, but no less evaluating the differences from a consumer perspective. I would agree that very few people seem to have a frame of reference beyond (a) stock and (b) XYZ fitted afterwards. While I've HEARD a few different after-market exhausts, I'd fall into the same boat of never having spent any quality time driving and living with them, making it really difficult to evaluate.
When I purchased my R8, I was fortunate enough that Audi decided to offer a "sport exhaust" for the final production year (2015). I ticked that box, of course. It's significantly more aggressive than the stock exhaust, but my suspicion is that it's nowhere near as flamboyant as SOME of the after-market options - many of which seemingly go for volume as their priority but may not prioritize tone and all-around drivability.
If you're just looking for something more aggressive than stock, but you still want to maintain a driving experience that works in all conditions, I'd recommend looking at the OEM sport exhaust. It's valved, and in the closed position, without romping on it, it sounds no more aggressive than a nice sports sedan. With the valves open, and if you're pushing it no less, it's a responsible loudness - meaning, you're never going to miss its presence, and it'll echo off all the hard surfaces around you, etc... but it also has a beautiful tone to it. "Mature" might be one word I'd use to describe it - it sounds present and rich, full bodied, and howling in the upper register, with a bit of rasp beyond 5 or 6k. I'd say it suits the R8's character well - the "gentleman's Lamborghini" if you will. But if you're looking for the loudest experience, and drowning out anything around you is the goal, then this won't be your option.
From the little I've experienced, if I was going after-market, I might consider Capristo - certainly loud, but I feel they're well refined.