We wouldn't have dropped into the EL, that's for sure. And also we would have fared better in LL, in the first months.
There is absolutely everything that indicates results would have been better under Koeman.
I think people tend to mix up Messi's struggles with PSG, Koeman's ineptitude and Xavi turning things around(albeit with some signings that turned the tide).
The only thing that you can say 'there's absolutely nothing that indicates' is how Xavi would have played Messi and how Messi would have impacted the team play under Xavi.
After all Messi would have been and still is better than any forward we have right now. If things would have went better for him at PSG opinions would have been different.
I don't doubt we would've been a better side during the start of the season with a highly motivated Messi after winning Copa. He would probably have carried us to a couple of wins.
But in the long run we needed to crash, we needed to rip the bandaid off. Board reacted in January and did some much needed moves.
A better start to the season wouldn't have seen Koeman get sacked and it would've lead to an inevitable disaster sooner or later anyway.
The positive in this is that we got Xavi early as we can still salvage some positives from this season.
Messi would've done more damage than good in this current side, unless he played as false 9. But then again, we'd be back to having players make up for his lack of workrate, one of the reasons why our defence has been solid under Xavi.