Fuel additives are technically banned by the CCR, but the ability to use any "commercially available" gasoline or E85 really undermines that. So, I can't just add whatever I want to my fuel, but I can still buy fuel from a distributor who has done that for me. VP Racing SR1 fuel would be one example... it's only 91 octane and is designed around SCCA fuel rules so that it "produces more power than any other fuel that has to meet these rules".
NASA's CCR does not have those restrictions, so we could even have people going further and using their more aggressive blends. The NASA Spec Miata rules have been changed to keep this under control... I think we should copy them.
"12.8 Fuel (Effective October 1, 2008)
Fuel usage is restricted to unleaded gasoline commonly found at retail pumping stations (Shell,
Chevron, Citgo, etc.). Octane is limited to a maximum of 93 (R+M)/2 as labeled on the pump.
Race fuels such as, but not limited to, ERC brand are prohibited. All fuel additives are illegal, per
the CCR. Note- event supplemental rules supersede this section."
We can really learn a lot from Spec Miata, for this issue, as well as others. As our class continues to become larger and more successful, we're going to have more people looking for an edge and we need to keep that reasonably under control. I don't have any reason to believe that anyone is cheating, but there certainly are ways to get more out of the rules than intended.