There seems to be some confusion about straight cut close ratio gears and when & why they are necessary.
First of all, forget the fact that the close ratio gears are straight cut. They would be stronger with helical gear form, but more expensive and the total sales volume is always going to be low, so they are manufactured with the SC form.
The reason the car has gears is because the engine can only give its power/torque over a certain rev range, so different gear ratios are needed for different road speed ranges, with a low gear being needed to pull away from rest and intermediate gears to enable the engine to stay within it useable rev range whilst accelerating or cruising or maintaining a specific speed on a level road or on a gradient.
The optimum gear ratios are, therefore, determined by the engine's performance characteristics, i.e. the graph of revs against power and torque, and the Final Drive ratio (FDR) is selected to suit this situation as well.
The original designers will have selected the ratios for the standard car and, luckily for we Mini owners, the competitions people at the manufacturers will have selected alternatives to suit different uses and states of tune.
The SC CR gearbox alluded to here uses the 4 ratios homologated by BMC Competitions Dep't for race & rally cars, and these are the ratios offered by the vendors of SC CR gearboxes. They are really intended only for engines with a narrow and high-revving power band used for competition. They were never intended for use in a road car as the disadvantages outweigh the advantages. The biggest disadvantage when a SC CR 'box is used in a road car is the need to fit a lower FDR to allow the vehicle to pull away in 1st gear, since 1st gear is high, 2nd & 3rd also being higher and all gears being close. If a lower ratio FDR is not fitted, the initial acceleration will suffer due to the need to slip the clutch a lot to pull away, especially on any sort of up-slope.
So, as a general rule you need a CR gearbox and a low FDR if:
The engine has a performance profile which gives a narrow power band (typically a Kent 286, BMC544 or hotter cam) and a high sustained rev limit (say 7000 rpm+).
It is to be used for competition (rallying, racing or hill-climbing).
You do not need, or even want, a CR gearbox if:
The engine has a broad power/torque band and will give its max power below 6000 rpm.
It is to be used as a road car including town driving
You want to be able to cruise it on main roads at up to 70+mph without it revving at, maybe, 5000 rpm at that speed.
You don't want to be frequently having to replace the clutch centre plate.
