Depending on the tune (remap), you're looking at about 50hp. The best bang for the buck is the Stage 1 JH Motorsports tune. I've been running this tune for a while and made 25 more wheel hp on a dyno. Stage 2, with a TCU tune is imminent and will release more hp, higher redline and a bunch of other improvements. Beyond that, they're looking at E85 (ethanol) which won't matter to anyone outside the U.S., possibly headers, a lightweight 1-piece flywheel and a few other goods. They also make a larger diameter valved exhaust which is, IMO, a better value than the Capristo.
Seat of the pants, it's a night and day difference. The car pulls harder to redline and there's a big bump in the midrange. A stock M4 would walk away from me before the tune and we're about dead even from a roll. I get him on standing starts. I currently have the AWE full exhaust (including downpipes), Eventuri intake and the JHM stage 1 tune. And a carbon clean, LOL. That alone was probably worth another 30-40hp at 80,000 kilometers on the odometer.
TVS out of the Netherlands seems to have some good information but I don't personally know anyone using their software on the RS5. They have a full engine/transmission tune package in stages and seem like they know their stuff.