I have often thought about this and even had made suggestions to some of my students about the left hand grip. One student who I suggested a split grip told me his band director 'would have a fit'. I asked him if his band director was a trumpet player - of course he wasn't so what does he know anyway. Personally I use a split grip - have since college days of playing lead at Ohio State. As I got older and my hands got bigger it just seemed to make sense since it was uncomfortable to cram your entire hand above the third valve slide. Younger player may find it easier to be above the slide because their hands are physically smaller and to split their fingers would hurt. Also - I am playing on a Olds Recording and the third valve slide is a trigger. . . pull on the ring to the third valve slide move out. - pretty cool. I'm surprised other makers didn't use this inovation. But really - how many low D's, C#'s and lower G's and F#'s do you play that can't be lipped down. I never though it had anything to do with more or less embouchere pressure. There was a fellow I met in Grand Rapids, Mich back in the early seventies who was a brass designer for Yamaha. Gene Pilczuc - sorry on the spelling - worked on this probelm and ultimately developed a stepped lead pipe. But I remember he was trying to make the slides rounder so they didn't have a hard 180 degree bend.