Mazda Demio.. XD. I did need to use nitrous, but with another couple attempts I might be able to do it without nitrous. You can get some wicked cornering rotation, easy power early on corner exit, and it's so small you can just chuck it into anything.
Pics don't show but widebody plus full aero, went down -1 in wheel size, wide wheels and wide offset.
edit: transmission should be set to 180. Oh and after some further testing, the Medium RPM turbo should be used, it has more torque which will help get you out of those tight corners on Tsukuba. Use TC5 just for launch, then turn it down to 1 or off for the rest of the race.
edit 2: okay, more testing, this time managed to take the gold sans nitrous. Same settings as below, but you want to add Weight Reduction 3, Body Rigidity, Racing Sequential Shifter and then set the transmission to 170. Medium RPM turbo with anti-lag, shift right when the bar starts turning blue so that you're constantly building boost. You'll top out at right at your braking point for the final bend, and you'll literally fly past opponents coming out of the hairpins.