Seems like it'd be a no brainer to bring both cars, but for whatever reason Mike doesn't seem to want to race at Road Atlanta.
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Might have something to do with what is unofficially referred to as the 'VanSteenburg rule'.

Several years ago ('99?) Mike won the ARRC in his ITS RX7. He was DQ'd in Tech for an illegal radiator setup. It was vertical. He won on appeal I believe, but the next year the GCR was updated to include language stipulating that the radiator had to have the same PLANE as the original.

Hence, the VanSteenburg rule. That coupled with Kip's DQ a few years ago during the BMW "Ballastgate" fiasco and I think you can see why Mike might rather stay home.