I've tried this several months ago when it was only possible with Chris' addon.
In my case, I had an increase in user registrations, but that didn't mean more posts or seeing them online.
Registered users don't see ads, but even that didn't encourage them to stay online.
I concluded that they're just dead weight.
I've taken away those popups, but the main page is clean and focuses on user registration/access, then I have UI.X with a top menu with both options always visible.
As user registration isn't in your face anymore, I don't have to carry dead weight and I'm getting a bit more from ads since guests spend a bit more time as they aren't annoyed with ads and popups, now it's just ads.
If you want to take a look, mtb.uy