I would probably skip the IP especially since you tried with the optimum line up. Why not pick up 8 oz versions of SIP and 106ff/fa off of the classifieds, it will probably come to a very reasnoable 30$, and that will be enough to do your two cars at least twice, if not 4-5 times over. Honestly, you are using so little of SIP, I have been using the same 4 oz on my hood/trunk/doors of an a6, and I have about 2 oz left, so a little goes a long way, don't pay attention to those detailing videos with guys slinging 1 oz of product on a panel from 2 ft away. Just use 4 small pea sized dots and you are good for a 2x2 section.
Just my .02cents. SIP is absolutely awesome on hard paint too btw.
IB