I am surprised to see the tracking you report as being .5 arc Sec. 35° above the Southern Horizon and .8 seconds of arc at 23° above the Horizon.
It would seem to me that if the Mach II mount with the encoders that almost completely eliminates all periodic error would have that much tracking error.
Does the atmosphere produce that much error? I share an observatory with a friend who has a 14.25" Newtonian f5.6, fork mounted with a 11" Byers gear on the RA and a SiTech II drive corrector. We have gotten guiding with PHD-2 of .21 RA and .27 Dec rms. using the Tik management feature. The Observatory is located at the SDAA dark sky site in Tierra Del Sol about 60 miles east of San Diego at about 3600' elevation.
I am waiting to be notified that the AP Mach2 I ordered last year. I have been thinking it would have more accurate tracking than what I already have access to. am I going to be disappointed?