I was just looking at what other seals are available that might reduce friction/increase performance. McMaster doesn't show anything for the 1.375" ID, however there is this teflon version for 1.5".
I wonder if it would boost performance at all, or if it relies on a smoother more perfect bore than what we might get from the usual tubing sources.
In trying to figure this stuff out, I found this design guide, which is just full of information. I haven't been all the way through it yet but figured you guys might be interested.