JVC has stayed with Silicon Optix Reon-VX processing, which, no doubt, gets improved each year or so, but there were no significant issues with the processing in older JVC projectors for that matter.
Next:
That brings us to the RS35 vs. the RS15. I've been working with both now, for a couple of weeks. It is, as you would expect. The RS15 is less than half the price, it's slightly brighter, but overall, looks and cooks the same, with the biggest differences being black levels, sharpness, and color accuracy. The RS15 (and its predecessors), don't have as sophisticated color setup, and I was never, for example, able to get my old RS1 as dead on as my own RS20...
Simply stated, the RS15 is a poor man's RS35. Save about $6000 and you settle for a similar look and feel projector that has the second or 3rd best black level performance around (assuming we count the RS25 and RS35 as a tie.) It won't be as dead on, it won't be quite as sharp, but side by side, there's no question about their lineage, they are so similar when viewing, until you get to really dark scenes.
Since the RS35 is the least bright (by about 10%) of any of the JVC's reviewed since the RS2, that's one slight advantage for the RS15. The other, of course, being price.
Finally that brings us to: