Edited in your version, thanks!
Also, reduced the save DC to 15+Damage taken since 20+Damage taken felt a little too much in retrospective.
I'm not sure I get your "liken it to a Massive Damage save" comment. I didn't use massive damage rules because they're non-scaling, and I wanted to make it harder to resist, say, a WildWurger's Beetle Crusher than a simple Gespent's Punch.
Isn't that what losing hit points from damage taken is supposed to represent? Joe F. Soldier is going to die, save or not, from taking so much damage. But should Bear Grylls ß, the time-traveling cyborg commando archmage who fights with dragons and wrestles with Cthulhu really be taken out by a piece of steel just because it's 100 times his size?
With the current rules, unless you're fighting a mecha much lower level than you, you'll generally only make the save on a natural 20. With the reduced DC, you might make it from time to time at lower levels, but at higher levels damage still scales much faster than saving throws.
Edit: Actually, having looked at the damage values, I think I'm incorrect, at least at mid levels. I'll have to run the numbers properly when I have the chance. I'm still heavily against SoDs on every attack regardless, though.