[bbcentral]

I only started using vBSEO in January. Personally I love it, I love how much faster my site seems to run now (a lot of features are disabled for guests to improve performance). I love how the pages that come up in Google from my forum seem to rank higher now, and only the important part of the page shows up in the search results.
I love how my forum has nicer URL's, although there are free mods to do the same thing (and vB4 will feature something similar).

Here's a little cheat, read this thread (I can't recommend this enough, it seriously helped me!):
http://www.vbseo.com/f45/your-site-r...o-first-23762/

Ignore anything that requires you to have vBSEO installed, but everything else is relevant.
Some simple changes like removing "Powered by vBulletin" from your forum title, having better forum descriptions, running Google Analytics and vBSEO Sitemap (Free) are quick and easy methods to improve your performance.

I'd say try doing as many of those things as possible and see if that boosts performance. Use Google Analytics Custom Campaigns to track different "Register" links and banners you have around your site, that way you'll know which links are resulting in new members joining.

Here's my forum running vBSEO, I have most of the options enabled:
http://www.bluesbrotherscentral.com/forum/
A lot of the changes I was able to do without vBSEO. I removed my boring default Welcome Message and replaced it with an image just under the navbar. It uses Google Analytics to work out how many people are clicking on it and signing up. I may change it to something different later and see if it has a higher registration rate, I can experiment and see what works.

So I guess all I'm saying is try all the other free options before you pay money to vBSEO. But even if you do pay for it, it's a quality product if that's what you need