the advantage is it gives more horse power! it is intended for use on a tuned motor, but would probably make a difference to a standard motor. (never tried one on a standard engine myself). but all the advantages are above 11,000 or so rpm and over a certian mph (probably around 60, i'm, not sure) as the box would need to be pressurised to get the best results.
the letterbox design forces air into the box and with the large capacity of it, the carbs will not empty the box of air, with a small one the carbs will suck more air through than is going in, therefore never reaching full pressurisation.
yes, a dyno run or two would be needed, as i believe on a standard engine you have to up the main jets to a 110.
when bikes are designed i think they have many laws to get round, emissions been one here. and that maybe why they didn't use the letterbox design, (i'm just guessing here

) another point to look at is, is that the standard design is 20 odd years old and if you look at most modern bikes, moto gp and road, they do use some sort of letterbox design as it is proved that the cleanest, stillest air is around 12" in front of the fairing and this is the best air to feed the carbs or injectors with.
as for figures, my own bike is 79bhp on a force fed dyno, there is alot of work done to it, but when we blanked the scoop off it gave 71bhp. so in theory it made a difference of 8 hp.
cheers
