I abandoned this test partway through once it became apparent there was too much RNG at work for decent science.
Firstly I altered the workshop from the previous run (which had no impassible tiles) to have only 2 passable and repeated the tests. I should point out that this is back to 1 boiling rock dropping. All other conditions are the same.
Freezing the game to spot the syndrome cloud was frustrating, I had the fps low enough that it wasnt a timing issue, but the cloud size is really random. One of the first runs created a cloud that stretched 3-4 tiles from the dwarf in some directions (across impassible tiles as it happens) and he was hit by the syndrome. The very next cycle produced a 1-tile cloud centered over the operator who didnt seem to notice. This was repeated, sometimes 1-tile. sometimes the centre and 1 tile in each of the 4 primary cardinal directions, sometimes a proper poof and crap everywhere. There appeared to be no trend between effectiveness of the boiling rock and the type of cloud I observed (not that anything changed between runs that we could use anyway).
Red spots are the syndrome cloud, its hard to see against the ground in my new mod. Passable tiles were the operator tile dead center and the center bottom tile. Gas is disrespecting this totally.
#1 ran 16 times. 9 dead, 7 live
#2 ran 18 times. 10 dead, 8 live
#3 ran 16 times. 6 dead, 10 live
#4 ran 19 times. 9 dead, 10 live
#5 ran 13 times. 10 dead, 3 live
#6 ran 19 times. 8 dead, 11 live
----gave up---
#5 is an anomoly, but not weirder than the one I got in the earlier runs checking boiling rock counts for effectiveness, just standard RNG.
Interesting side-note. Due to the occasionally properly mushrooming boiling rock you can actually get freebies on these syndromes. After I gave up the tests I quickly reloaded the testfort and drafted 9 into the militia to stand round the workshop while the 10th set off boiling rocks. I only tried this a couple of times and cant rule out that a bystander can be hit while the operator is not. However on one beautiful occasion I managed to hit 5 guys with the same rock syndrome (including operator). Thinking about some of the places these syndrome rocks are used in major mods im happy I know this. The reaction cost for producing a golem or a mage or something isnt so hideous if you get a load of freebies:p Though if said reaction has a fatal fail-chance you might start a spiral. I certainly sent a lot of dwarves into insanity, its the only excuse that got them off the DOOM reaction duty.