616: 6-8 hours at NCM
617 non-turbo: 5 hours at Road Atlanta, which has a lot of WOT
617 turbo: 2-3 hours at Road Atlanta
From a racing perspective, the 617 turbo is a legitimate C contender, while the 616 is a pretty good contender for the cool trophies, but requires some pretty good situational awareness and traffic management skills to operate safely.
I did not race it with the 616, I missed that race, but I did with both iterations of the 617. The turbo is the most fun, but the non-turbo was a nice balance of long driving stints and enough speed to not be horribly dangerous to myself and others. Put some sticky tires on it and it will carry decent speed, and baby the brakes.
If you've got a 300TD that you can cage, then I highly recommend it. Just pull the boost reference line at the back of the intake manifold and cap the fitting if you want the non-turbo experience. We ran it like that for 1.5 races before we figured out there was supposed to be a hose in there.
Everybody grab your brooms, it's shenanigans!