It depends on the numbers we're talking about. If we were to tweak what we currently have (so the final rank would require 200,000 posts), I'd consider that to be overkill. All that you'd be doing is making the upper tier of ranks next to impossible for most people to reach, even for those who are fairly active in Spamville. That would make the whole arrangement pointless, since, in particular, it blatantly ignores the people who make productive posts, which the ranks should truly be for.
If the highest rank was 10,000 or 20,000, I'd be fine with that. So long as posts are only counted in every forum but Spamville. All that you'd have to do is add up all of your posts from the other forums, and that would become your post count. I don't think I have even a thousand posts outside of Spamville. (Which is funny, since it's the complete opposite for me over at MSF, where the vast majority of my posts are outside of the Muffin room.)