I have just over 200,000 posts and host on shared hosting still (but it's a cloud/cluster type service). I don't use any caching and can't do any server tweaks because it's shared.
In particular I'd like to know what kind of speed increase, if any, hosting on VPS or dedicated might bring (from your real life results looking at page generation times).
Pages on my forum typically take 0.2s to 0.3s to generate.
The fastest page is the What's New with no new posts - 0.14s
Forum home page - usually 0.2s to 0.3s
Entering the forum thread view for the biggest forum (500+ pages) - 0.3s to 0.5s