The first code seems to disagree with my system. I entered it correctly, and checked my entries to be sure, but the game just doesn't load, for some reason.
The second code works... interestingly. When I turned the system on, my Model T was set to setup A, and the wheels were of a smaller size. I switched to setup B and the wheels resumed their normal size (or, at least something very close), and the car turned into an axe murderer. It seems the only way to get around a turn without risking the chassis' natural bit of oversteer turning into a 60 RPM death machine is to use left-foot braking all the time. My theory: The lift-throttle oversteer is caused by soft front suspension, and the extremely unpredictable on-throttle behaviour is caused by soft rear suspension (Will the weight transfer backwards and stabilize, or will the wheels spin, killing rear grip and propulsion?
Both?! ). My solution: harder suspension. Also, you said that you applied FGT LSD? Maybe the settings are impossibly low or something, but there is no limited-slip effect whatsoever. I actually did 1-wheel burnouts to test this thought, and confirm it did! The transmission, though, was pretty nice. It never occured to me that it was possible to adjust the final gear of the Model T's 2-speed tranny, and it works very well. The insanely flat power/torque curves of the engine don't at all require many gears, and since I don't have to use my concentration shifting, I have all of it for handling the beast! Even so, it's not enough...
Which version of GT4 are you using? That may be why our Set A codes don't quite respond in a similar fashion.
EDIT: By the way, the pic in my avatar was done in stock, 20hp, 50mph trim. It seemed relevant.