I think that there are two or three distinct questions in your post, so I've broken it up:
My friend and I have been having a disagreement about what discs are best for max distance... I think that understable discs have the potential for more distance, and he thinks that overstable discs are better for max distance...
When talking absolute max uncontrolled distance, fast and understable is better because it allows a skilled player who hits their angles just right to maximize the glide phase.
...We both throw around 300' to 360'... he thinks that overstable discs are better for max distance, because most pro players throw Destroyer type discs. Does anyone have any input about what discs/weights would be best for our distance/arm speeds?
Most pro players throw more stable distance drivers because, given the choice between
A) Throwing 450' and landing in the correct 30' landing zone, or
B) Throwing 550' and landing who-knows where,
Choice "A" will give them better scores *on the golf course*, and a more stable and predictable flight can be achieved with those overstable discs. That said, they will use their less stable Destroyers (lighter, more beat in, or flippier plastic type) for longer and more open holes, so even they go for more understable discs when "pure distance" is needed.
However, if you're throwing 300'+, a Destroyer probably would not be a useful disc for you. Try something like a teebird for a similar straightish to stable flight for your arm speed, or Thunderbird for something slightly faster. If you're getting above 360' then maybe start trying destroyers, but even then I think they're designed for 380'+
