A QUALITY poster posting in page 3000 would still produce a quality post. Mega threads don't breed shit posters or spammers, it only helps to hide them. If signature no longer appear on mega threads, shit posters will have no option than to polute more innocent threads.
There's no how a mega thread would be as decent as a two pages thread, for the love of soccer I visit those mega thread and read, when I see such shit posts I'll just smile and move on because I expect such a thing to happen.
I guess it depends on the mega thread. One's like discussing soccer, make a lot of sense. However, there's a bunch of mega threads that are basically obsolete since every single thing that could be discussed have been discussed. Yet, users continue to participate in discussion, mainly the build their post counts. This is most evident in the Altcoin Discussion thread, but it happens else where too.
Limiting signature past the 1000 page probably wouldn't help all that much. I think the solution would be to better moderate these mega threads. Whether that's through increased manpower, community helping out or being a little more restrictive on mega threads. The latter probably being the least ideal solution.