The SI would be an excellent choice and Megan Racing already has a race valved suspension with the proper spring rates available to the general public so we may begin to see them pop up along side Jason Tipple's SI build next season.In a reddit discussion, Alex Piehl believes he could make a newer WRX work better than his '14 (less weight, better something else). He also thinks the VAG platform is solid, but I think the fwd GTI is limited by a lack of 9" rims and there aren't enough TT Quattros running yet to separate the car from the drivers' skill. Thorpe's Golf R was in the hunt, about 0.5 sec behind per day, hot on the heels of the TT Quattro, so I think it's no worse of a choice than the TT Quattro despite the DSQ.
Alex Piehl also likes the 2017+ Honda Civic Si. It sounds like a couple people are building them for the 2020 season. The car is one of the lighter ones, 18x9" rims are available, it already has a good LSD and superior suspension geometry, but I heard they still might be camber-limited? And there's the stuck-between-gears problem since 2nd gear is only good for ~55 mph, so it will likely take a really good driver to put them in the winner's circle on National-level courses. Still, Honda contingency should pull in those good drivers.
Krauss' Talon TSI was not even close this year after winning during last year's weather debacle, but he also didn't attend any big events, so rusty skills might have held him back. He was so far back he did not beat the winning GS time.
Then there's us with the FoST… it's clear you gotta throw money at this platform compared to the Honda and GTI. Neither has anyone proven 265 tires on 9" rims are enough for battling awd. I'm still sticking with GS for the next two years, working on my driving, but in the mean time I continue making plans and figuring things out.