Fast guys can ride anything fast, it's the little tweaks to their bike that allow them to go even faster. Think about it, if you can ride your GG at a B level then you can jump on a old XR400 and go faster then the bike was ever designed for. One of our A riders in FL had several new bikes but LOVED riding his clapped out, bailing wire and zip-tied together 99' KDX 200 and I could barely keep him in site on my new at the time 08' 300 (mid B rider at the time).
At West Point, TN National Enduro this year all the GG's looked basically stock and the KTM's and Husa looked to just have a little extra bling, maybe triple clamps, and black custom wheels, but also mostly stock. Of course all had the standard protection added.
I think the MX bikes have more little "customs" to them then the off roaders.