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I am not aware of the long/short leadpipes but it would be easy enough to make a longer leadpipe.

That being said... I've always thought the bit was for ergonomics rather than for intonation. Many of these big horns were played in stands like 'tuba tamers' and a bit made the mouthpiece position flexible.

As far as intonation goes... there's no difference from using a bit and pulling the main slide out 3/4".

If it needs a bit the receiver is jammed right next to the bell stack. It comes around at a straight angle, but only at middle of the bell. Without a bit the valves are at a bad angle with your elbow pulled back, and visibility is bad because your face is right up in the tuba. If you need a bit you can tell.

I found the bit to help intonation too. Who knows why, I guess the leadpipe needs a certain length before going oval shaped for the valves.