I've been working with Stilen on my AP vibration issue which we think is more of a pad transfer material issue but when I had mentioned the same issue as the OP has had with slow speed (3~5mph) pad to disc contact rubbing vibration issue, they sent me the spring/rod kit that and I think it helped. Never mentioned the rubber pad behined the brake pads.
Right now I have Hawk race pads on to do some disc face cleaning and do not have the spring/rod kit installed and again I'm feeling that same vibration as the OP mentioned at very slow speed.
Soon once I repaint my calipers and change out the piston seals with the street seal kit Stilen sent me, I'll install that spring kit again and see if vibrations are all gone...I'm praying.
Have people been experiancing more vibration issue's with AP BBK's compared to StopTeck's and Brembo or is this a universal issue?