Tim,
I really doubt you would be able to tell the difference in actual pool operation.. None of mine are mounted vertically and the one at my house has about 1" of straight pipe before the cell.. As far as I can tell, they all work the same....
In my mind you are trying to make this a game of darts, where only bulls-eye counts.. In actuality, it is more like a game of grenades, close is close enough.
My view is if you can make it like the manufacturer recommends, then do it.. If not, then just do it the way that works best for you.
Thanks,
Jim R.