I think he wouldn’t help them as much as he’d help us. Despite actually winning these games last season, it always felt like we’ve had a problem of breaking down teams that just sit in and defend and someone like Messi would really help unpicking the lock in those sorts of games where you’re in the 70-80% possession range.
That wasn't their issue last season though. They dropped points against the weaker teams because of their defence, losing 3-2 to Norwhich or drawing 2-2 with palace etc. Think there was only one game against the bottom sides where they lost points and didn’t score, 1-0 against Southampton.
As for the bigger games, for me, I’d question how much their is to be gained in what he can do around the goal compared to what you’d lose in terms of work rate and control if he was playing in midfield. If he’s playing up top, obviously brilliant when you’ve got the ball, see Barca away last year, but when you’re having to defend and looking for a quick release, he isn’t beating anyone in the air or over the top in a race anymore, so can become quite isolated, see Barca at home last year.
I think he probably would get them more points overall, but no where near as many as if they used that money to sort their defence out. Having said that, they’re a PR stunt of a club and what’s going to get more attention than signing Messi?