Are you going only with 4 gears or shifting with all 5 gears? Because if you are doing all 5, that 212 speed is invalid as the car originally had a 4-speed gear box, car simply can't reach it's 212mph speed. It's missing something. Going on the mulsanne straight. I can't reach any of that speed at all, even if the gear ratios are wrong, the car feels absolutely awful when I try to accelerate.
I used all 5 gears. Again, unless you have irrefutable proof that the car which set 212 mph only used 4 gears, this is all getting too technical for the level of game that GT is.
Do I wish the Mk IV is better balanced to be more competitive with the 330/XJ13? Absolutely. Do I care that the acceleration and top speed numbers don't match exactly with real life? Nope, there are bigger problems with GT physics to be fixed first.
As proof of concept, I tried tuning the gears only with 4 speed. Stock power & weight, minimum downforce, La Sarthe no chicane arcade mode time trial. Set top speed to 450 km/h, then Final gear to 2.900. Top speed is 330 km/h (206 mph). If we could reduce all downforce to zero I have no doubt it would reach 212 mph like the "claimed" real car.