Since it's inception, the 727 has had a problem with rear servo and rear band breakage; this is due to the fact that line pressure is about 3 times higher (around 300 psi) in Reverse than in the forward gears. I believe that they purposely made the rear servo feed passage smaller on most units to limit the flow to the rear servo to help prevent problems. I also believe they made the passage deeper in HP versions to aid servo release.

In later years ('71-on) they did make the rear servo feed orifice in the steel separator plate smaller to limit flow and most kits instruct you to enlarge this orifice. The ideal setup was used for a few years when they used two orifices in the steel plate (one large, one small) to feed the rear servo, the large orifice was blocked with a check ball during apply that fell out of the way during release but they (unwisely IMHO) dropped this feature.

The deeper passage did carry on into later years due to the smaller feed orifice that serves the same purpose as the shallow passage.

Since the rear servo isn't applied in Drive the size of the passage/orifice has no effect on the 1-2 upshift in Drive.

The rear servo plays no part in 2-3 upshifts.