Its because the M21 has such a high first gear ratio and the 253 will really struggle with it if you have a tall diff ratio as well. Yes because they are not as torquey as a 308. M21 was the sporty option with closer spaced ratios.
So you have to go with a lower ratio dif and then you loose out on highway economy as all the gearboxes have 1:1 4th gear.