New to Mercedes Benz. I have a 2016 g63 amg I just got. I have a long history with GM LS motors and have built and tuned my own cars.

I use HP Tuners software.

what are these beasts capable of? Assuming I keep the stock turbos, swap the full exhaust from the heads down back what can I expect hp wise? Are there any supporting modifications that are necessary?

Off-road tires with aftermarket wheels are a popular modification. My only caution is if you have the speed delimiter, your car can exceed the speed rating of the tires (as no off road tire can be rated for the delimited G63 top speed as far as I know).

700HP with stock turbo, tune, stock exhaust manifolds, catless downpipes, and hi-flow muffler. I think to get much more than that you would need water/meth kit and then start looking at turbo upgrades, etc. Not sure what program you need, but I know my mechanic had to buy a specific software set to be able to get into the Mercedes programming. He didn't do the tuning, but he has access to all of that with whatever program he purchased. Not sure if HP tuners can crack into their system or not.