I found a pair of grips in the junk parts bin at the local shop this morning (2 pair actually) they were pro tapers, but called the "Rockstar" and were thinner than the renthals. I bought two pair, 9.99 and installed only the throttle side grips using the simple green and they slid on in 5 minutes.
The renthal's I tried were the medium compound (gray color) and by comparison are very thick when compared to the pro tapers. I am guessing the compounds make a big difference. The pro tapers are single compound and I compared them to double and triple compound at the store and the single was the thinnest of them all. Maybe the ones I got were too hard to stretch comfortably... The pro tapers feel fantastic, the only area that throbs is where the sutures are!