Whilst I would not have the nerve to argue with Matt about setup, 50/50 Avgas/Unleaded is less likely to seize than straight unleaded if the jetting is less than perfect for the conditions.
It will also allow you to increase compression and otherwise tune further with less risk of detonation.

The only true lead additive I know of is Tetraboost, more than one guy on twostrokes.com has successfully run a leaded race fuel GP 250 on it.
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