The difference in magical damage received between a DRG and a WHM is roughly 10%. Tested it on 500 hits on water elementals in Urth's Gift. (Average of 127.5 on WHM and 141.66 on DRG. -10% damages on WHM)

It's not really a liability but you do require extra heals, especially after megaflare. The cure 3 isn't going to top you off. I also died with 5.8k hp and stoneskin+succor to megaflare because I was out of sacred soil and didnt think I'd die when people are living with 4.xk hps.
Let's say a skill is enough to kill you in one shot (haven't tried T9, can't say!), so... 6k damages?
A DRG would receive 6k while a WHM would receive around 5k4. You might not survive, but I'm pretty positive he wouldn't either, because of his lower max HP.

If he was in Sacred Soil, which grants an additional 10% damage reduction, the WHM would only receive around 4860, and thus might survive. Now if you were in Sacred Soil, you'd receive only 5400 and survive too. Barely, but hey, alive is alive.

Also, we have Second Wind for most of these cases (ohai self heal for ~1k), and could use X-Potion/Hi-elixir. A nice mantra at the right moment could help as well.

The fact that some dragoon cleared T9 tells me it's not such a big problem. The only problem would be if you got one shotted and had no way to survive, which is not the case.