Pantheon main, I also think he does plenty of damage, go AD rune page with scaling MR glyphs and armor seals, and you are a beast early game. Then for items anymore I almost always go Hydra that's +80 damage plus an AOE on basic attacks, means I can quickly clear and push a lane out, then my 2nd damage item if I'm laning depends on what i'm facing I'll go either Maw, LW, cleaver, BOTRK, or pretty much any other AD item short of BT or IE. If I'm facing an AP lane I'll drop Hydra to my 2nd item and get the Maw, if I'm facing a team with 3 tanks I'll start Cleaver and then go LW, it's not much total damage but I need the armor pen to cut through the armor. Occasionally I will buy a 3rd damage item but most of the time it's just 2 then boots (after I finish my first item) and 3 defense items, since panth starts falling off late game he needs some defense to stay in team fights, or he won't have time get all his abilities off before getting crushed especially if he wasn't able to ult into the fight.
For jungle he doesn't do nearly as much damage since you have to start machete, so I keep my laning masteries, I don't even use jungle masteries, if you do then you do almost no damage when you do gank early. I go chilling smite warrior, and hydra with mobi boots and the rest defense. I would also suggest taking some kills when you gank. When I play panth jung I try to spend as little time in the jungle as possible, I get my buffs and then gank, I clear raptors and wolves and gank, I back after a gank, oh buffs aren't up time to gank, dragon, gank. Abuse the fact that you are one of the strongest early game champs there is
Everything above seems to work for me as many of my panth games show, but I probably have 400 total games with him and over 100 ranked this season. So I think I pretty much understand everything about him, other than the days where I just can't hit somebody with his ult. You said he doesn't need any changes with his R, I think he needs an OMW ping, though I would probably still spam pings when I ult anyway to get the message across to my team not to back out.