I agree with Steve on the subject of the MC18 gear selector. Better than 21/28. More positive and less missed gears in my experience.

Also, gear ratio's that suit some people/engines may not suit others. Fitting the closest box you can find all sounds very good until you realise that if you're after top speed you can't get away from the lights without frying the clutch, or if you want a quick easy getaway then the thing hits the red zone in top way too early. My 300 street bike has an F3 box. It's geared for about 60mph in first and will thankfully pull it without too much grief. I wouldn't want this gearbox on a stock 250.

Yes, the 28 primary (2.5:1) puts more torque on the gearbox than the 21 primary (2.36:1). I know of at least three (HRC supported) race teams back in '95 & '96 that were using the 21 primary gears on their 28's.