I personally agree with most of the stuff SentySent mentioned in comments regarding the cannon. Currently unless the soldier is really slow it doesn't make much sense to use rifles over cannons. They do more damage per shot, more shots per clip and can be fired the same number of times per turn.

Making cannons more focused on support rather than damage per turn might be a good solution. Though ideally it would be an optional setting.

I second SentySent's comments on the grenade perks...Heavy Ordinance and quartermaster are stricktly better than their counterparts in the support tree.

A perk I don't think is working out too well is Centre Mass. +1 damage on all shots? That's a no-brainer. Everyone gets that, no matter their specialization. There are a few perks like that in RPGO, ones that can always be picked and are good for everyone, like Deadeye, Gunner and to a lesser degree Grenadier as well, but Centre Mass is probably the most general-purpose of them.

Well it's still a trade-off. I might increase the AP cost to a point where it only makes sense to pick Centre Mass if you're 1) using a weapon with low base damage or 2) shooting multiple times per turn so you get maximum mileage out of it. Tbh I don't think the perk is a no-brainer even now, especially not as weapon damage gets higher. For a T1 Bullpup it might be very strong, boosting its damage by almost 30%. For T2+ cannon on the other hand, the increase is less than 10% effective damage.

All other perks you listed are obviously good for soldiers who shoot a lot and are using other perks that synergize, but there's still an opportunity cost. Personally, I almost never use gunner because I usually have a free utility slot for ammo anyway on soldiers who are primarily shooty guys.

What other RPGO addons are you using by the way? I think that with more options, soldiers become less and less similar.

I'm only using yours at the moment.

Part of my argument about no-brainer perks is that in my opinion AP is too plentiful. It's not as if I'm not having to make choices, nothing on that level, but you get so much XCOM AP that you can usually afford to buy something like Gunner as an extra for everyone, even with something like a 40-man roster.

That makes sense, though I haven't had that experience. Are you using any mods that alter the amount XCOM AP gained in any way? Possibly this one https://steamcommunity.com/sharedfiles/filedetails/?id=1123788033 ?

If not, perhaps the overall AP gain in RPG Overhaul needs to be looked at.

EDIT: Also (shameless self plug), if you want more options for grenade-throwing/launching and shooty soldiers, have a look at these plugin mods (if you're not using them already):

https://steamcommunity.com/sharedfiles/filedetails/?id=1551577550
https://steamcommunity.com/sharedfiles/filedetails/?id=1521073823
https://steamcommunity.com/sharedfiles/filedetails/?id=1511123422