I asked myself a similar question last year and did a comparison of lap times between ST3 cars in S-Tai and the M235i Racing cars in VLN and Creventic. Turns out the lap times are very similar in terms of percentage of GT3 lap times in the their respective race series. GT4 cars are a few seconds (4-6 typically) faster than M235i Racing cars when they race together, so it would follow that GT4 cars are a fair bit faster than ST3 cars.Also with the GT4 class. With cars like McLaren, plus the M4 and AMG GT to be ready for GT4, it's going to be cheaper than GT3. I also wonder how close a GT4 car is in performance to ST3.
Can't wait to see how the diesel Mazda3 goes.Some team announcements:
ST-X: Y's Distraction GTNET, led by Kazuki Hoshino
http://supertaikyu.com/teamrelease/data/2017/gtnet.pdf
ST-3: Denso Le Beausset Motorsports, with drivers Koki Saga, Yuichi Nakayama & Kenta Yama****a
http://supertaikyu.com/teamrelease/data/2017/lms.pdf
ST-4: Cusco Racing, led by legendary drifter/racer Eiji "Tarzan" Yamada
http://supertaikyu.com/teamrelease/data/2017/cusco.pdf
ST-3 and ST-5: Team NOPRO, with a diesel-powered Mazda3 and Mazda2
http://supertaikyu.com/teamrelease/data/2017/napro.pdf