yeh i think any decent coilover setup with matched shocks would be better than the stock setup. stock is too soft anyways. how much was your tein SS? i'm skeptical to believe that its using the same pistons as the HA's as there was a MAJOR difference in performance between the NA and HA's.
ive been using the HA's for the last 4years and have never needed to get it rebuilt. (hard canyon street driving, no racing at the track). if the SS ARE using the same pistons as the HA, i'd go with the SS and save some money.
about pillow mounts. yes, it does benefit to have them if you are a hard corner carver and /or race at the track/autox. if you are only using the car for cruising around, no need....BUT it does make the installation 10x easier if you have the pillow mounts because you will not have to use a spring compressor to dissassemble the factory ones (unless you buy the factory mounts all separately). there is more audible shock noise with the upper pillow mounts, so if noise bothers you go without them.
personally, i use pillow mounts on both lude and wrx and not on the G20 as its a commuter/shopping car for the wife.