Yea, unfortunately many people don't know how to tweak their ipb's or vbulletin's :/
if i had to take sides, i probably would say its better for LP to not allow the large forum pkgs. personally, back in the day when IPB still had a free version [now its 15 day free trial] i got it so streamlined it was about 20% less server load then phpbb. I had cut a bunch out of it, but still liked it over phpbb because of its better search abilites and INCLUDED sub-form possibilites.
How does this pertain to original post? Big forum boards requres big forum pay

If you are willing to hack up the phpbb quite a bit, then I'd recommend doing that. Otherwise, you could possibly go w/ a CMS and use their included boards. If you need any help with either issue feal free to let me know. [sorry about capitilization and lack there of...i wish they never existed.

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Ryvin