Well, I'm halfway through professional mode now, and I'm unfortunately a bit disappointed.
The good thing is that Pro difficulty is obviously designed for the player to have upgraded weapons (but not infinite weapons), and the secret weapons seems to be well balanced for it. The bad side is that it's difficult in a way which can be extremely frustrating. The differences I've noticed so far are these:
-Enemies are faster to get back up after being knocked down (doing the ground melee attack becomes almost impossible)
-Many items are removed (for instance, in one the shanty town buildings you'd normally find 1 red herb, some handgun ammo and 2 grenades, those are all gone)
-And the one huge difference: almost all enemies will kill you in one hit. And the dying state timer is insanely short. So short that you literally have to be standing right next to your partner if you want to be saved.
It can be fun at times and it's definitely a huge challenge. But it's also very frustrating at places, especially when you fight enemies who use molotov cocktails or crossbows. Some of the bossfights becomes very frustrating as well since missing one single QTE means game over. I don't think this is a mode you'd want to play with strangers because you'd easily become a huge liability unless the both of you work closely together and have a lot of patience.
I'm having fun with the mode (besides a few super frustrating boss fights, like the one in 3-3... if anyone has any tips on how to easily avoid those QTEs, let me know), but it's not a mode I'd be tempted to play again. And it just reminds me that Capcom really messed up the balance in the game. They never should have implemented persistent inventory across all modes. They should have reserved that for new game +. I think that's one of the reasons why they made Pro such a weird difficulty. They knew players might play the Pro difficulty armed to the teeth, so they made it difficult by giving enemies basically the same advantage that you have.
One thing which is a big disappointment is that I can't play the game the same way I played the demo. I enjoyed playing the demo with random people and see how their playstyle would be. That's impossible to do in the full game. If I host a game or I join someone else's game, there's no chance we have balanced weaponry. One of us will make things way too easy or way too frustrating. And jumping into random coop games with Pro difficulty won't work well (most people are just chickening out and playing that mode with infinite ammo anyway).
I checked out the settings for splitscreen and I noticed they have actually implemented 2 versions of it. If the second controller is logged into a gamertag, it works the same way as online coop. If the second controller is not logged in, he becomes a "guest" and everything is based on the host's savegame (which means both players can freely trade weapons, gold won't be doubled, and the second player will control Sheva with the weapons the host gave her). If they had added the guest option for online play too, that would have been a good solution. That and removing the stupid persistent inventory of course.
If I ever replay the game again I'll definitely do it starting with an empty inventory. And if I play it coop I'll make sure my partner does the same thing. At least Capcom didn't make any mistakes with mercenaries mode. That mode is still balanced and a ton of fun to replay.