My problem with this is that you could argue the same thing for a JC120 or Mark I as well. But I think the more you modify the tone with the pedal, the less of the amp you hear and feel (not that a JC120 has anything you could call "feel" ).
Philosophically, how far from the native tone can you get before you lost the amp's magic qualities?
These are good questions and worthy of discussion, but to be honest, I don't consider enough much of an expert on metal tones to be able to give a knowledgable answer. No doubt at some point you're hearing more pedal than amp.
If I was a metal player, I'd get an Archon. But I'm not, and Texas Minor Blues hasn't mentioned a need for metal tones, hence my recommendation of the HXDA.
The reason I posted the video link was because you asked if it could get a tight, gritty, gainy rock tone, and didn't specify more. To me, that's pretty tight and gainy. I'm sure tight-gritty-gains means a lot of different things to different folks, right?
What the amp's gain limits are, I honestly don't know - I haven't been there and done that. At some point, some client will no doubt make me find out though!
I very rarely listen to metal, it's not that I don't appreciate it, but I'm less familiar with it than other styles and tend more toward other music when playing.