The turmpet length is to do with pulse tuning the inlet. As revs increase the pulse with get shorter, and hence a short set or trumpets suits higher revs. On my TB's I run 90mm trumpets as that's the biggest you can fit in to a std engin ebay, but 120mm is meant to be the optimum set up.

But as Dan says it's makes little noticable differnce, it's for race teams that want the extra 1% or whatever it would offer.