I never got understeer from any of the various tires and replicas Cobras I've owned/driven with any tires until,,, I recently went to Goodyear slicks. Now I have a slight tendency to understeer, but the good news is there is no way the back end is stepping out!
I have to induce throttle over steer if needed.
I've run 16 and 15" tires. Auto Cross DOT tires, BF Goodrich TA's, Goodyear GT-11's and Goodyear Bill Boards. Always the classic sizes, 245 front, 295 rear sort of thing. ALWAYS the rear end would come out first under hard cornering conditions (over steer).
The reasoning behind smaller tires is all about "matching the traction" front and rear to arrive at a more nuetral cornering condition. I find I prefer a slight understeer rather than a big over steer.