I think you will notice the 1st gear ratio change if you fit the standard T9 box. My MT75 box also has a similar short 1st gear and it is useless unless you're taking a fully laden Ford Sierra towing a caravan up the side of a mountain.
It's why I have been looking at the Mazda box which has a good spread of ratios for Sports car use and still with a good overdrive in 5th.
I now know that the Mazda is not an easy fit in your car so if you are going to fit the T9, I would really suggest you look at the alternative ratio set which whilst being quite expensive will be a much better driving experience.
I'm also not sure about the numbers changing the axle ratio will make if the original is 3.97:1
At 3000rpm the difference will only be about 100 rpm, not 300 rpm. It's only about 3%
The final drive on mine is 3.73 and that would give about a 300 rpm difference at 4.1.
