Thank you Bob, Croc, John, and Kitcat! Quick thoughts on each, in the order I tried them:
620R S3 with carbon Tillets: this was the first seven I've ever sat in. Fitting like a glove is exactly the right analogy (5'8"/170lbs). The carbon tillets were super comfortable. Definitely understand what a small toe box is now!
310S SV with leather seats: felt…sloppy? Granted this is quite the contrast after the previous S3/carbon, but I was surprised at how little they hold you in place. By comparison my S2000's seats feel much more secure.
G7 SV race car with the FIA Tillet "Big boy" XL seat: basically went for a swim in a race car.
CSR race car ("Catzilla") with standard FIA Tillets: solid. Gave me the best comparison to the kit in question as it's the same configuration (including lowered floors). Visibility was okay, but I'd definitely prefer to be higher up. Easy enough to remedy later.
Thanks again to Bruce for allowing me to test fit all these combinations. Prior to yesterday I was driving myself a bit crazy trying to choose from all the options available in Caterham's configurator. After having sat in a few I'm realizing most of that stuff doesn't really matter — it's going to be fun no matter what I end up with. Which, by the way, is this kit