We just had a little 'school' with the DOT inspector, a week ago Tuesday. He says you WILL get fined, and the Feds really bust your cajohnes.
His advice to the farmers, was: If you have to run non-tax fuel, in an emergency, write down the estimated amount you put in. Then go to your fuel supplier, and pay the tax. They will give you a tax receipt, which must be carried in the vehicle.
He also said it was not entirely kosher, to have a big stack of fuel tax receipts in your rig.
Basically, do it if you have to, but be sure you get the taxes paid, before you get caught.
I live exactly in the middle of BFE, and they had a big surprise check, at the sale barn. Caught about 8 people running it. Every one claimed they used it in an 'emergency'
. They all got to sign a Notice to Appear.
I have no idea of the performance benefits. Except making your wallet stay a little thicker, temporarily.