In the vertical descent under a canopy case (17 knots as quoted = 8.75 m/s), my estimate is that the "crumple zone" is 0.5m - essentially the height of the landing gear. Assuming that the crumple zone crumples evenly, you are confronted with an acceleration of 76.56 m/s^2 or 7.6G. Very survivable, as multiple CAPS deployments have shown.
Possibly conservative as Cirrus say there is an additional crumple zone between the seats and the belly. Moreover to compare like with like you are assuming the landing is on solid terrain in one instance (with CAPS) but through bushes etc in the second (without CAPS). A Cirrus landing under CAPS onto a hedge will enjoy the same benefits as a FL through a hedge. Finally the energy vector is preferable vertically than horizontally as previously discussed.
All in all I agree it is academic however. The evidence is many FL are very successful and with some luck that you dont hit anything too solid they will probably work out fine but without luck the outcome is poor. With CAPS the outcome is more heavily weighted in favour of the occupants across a range of landings, the weighting improving at night or with low cloud where a conventional FL becomes more of a lottery.
So the argument in favour of CAPS is with CAPS you are pretty assured of a landing with 7.6G (to use your figure) or without CAPS between 45G and 4.5G without really knowing which card you will get dealt?
I quite like the idea of reducing the chance element as much as possible which is why I think CAPS a good idea all other issues aside the more especially if you intend to fly over a variety of terrain, at night or with low cloud.
I'd be interested to see more figures regarding stall speed and forced landing survivability, and I'd have thought it would help in deciding when to pull the handle.
I am not sure. With CAPS the pilot is going to make a quick assessment of whether he feels reasonably certain he can complete a successful conventional FL. I guess if there is an attractive field and the pilot is comfortable his skills are sound that might be the preferred option.
However, at the back of his mind he is also going to be aware for the above reasons that he is more assured of a successful outcome with a CAPS deployment.
I agree that the slower the approach speed (in types where CAPS might be installed other than Cirrus) the pilot will factor in the possibility of landing in a much smaller field or accept a landing site that he might judge unsuitable in a Cirrus.
Whenever I have asked a pilot to demonstrate a PFL I am conscious that their skills are more often not as sharp as they imagine. I suspect most FLs end up than being less than the perfect ones we imagine. However I am pleasantly surprised how often the outcome is reasonable. I have never been certain whether this says more about the performance of us pilots when push comes to shove or the resiliance of the human body and the aircraft we fly.