I have run a very niche site for almost 5 years. Was vb from the start (versions 3.7x until 4.x.- with suite). The site is at www.pebforum.com . For a few years, I had the first google results for my main keywords (physical evaluation board). Over the last, say, 10 months or so, I saw some dropping by a few results. Then, about two months ago, I "caught" a vbseo related sql injection something or other, and google dropped me altogether from search results.
On March 12, I finally migrated to XF. On March 14, I was up to the second in google results. Then, On March 17, I am back in first place for my keyword results.
I am always somewhat reticent to make apple and oranges comparisons on SEO. That caveat aside, I am fairly well shocked that I moved back to first place in the google results so soon after migrating to XF.
This is all aside to the general great feedback I have had from my members since migrating.
XF might not be best for everyone's needs. However, I am finding that it has been a great move for me.