If memory serves those guys toxin is in their hairs/spines right?
I'm such a freak that I've been sort of consistently disappointed by the pain of the stings of notorious insects. Never even seen one of these guys though, sounds gnarly.
Dunno if I'd try and piss off a velvet ant so's it'll sting me, but at the same time there's this sick desire to try it out. I've read the velvet ant has the most painful sting in the American southwest. The Tarantula Wasp's sting didn't hurt very much (a close relative of the velvet ant) , but that may be because she didn't have time to really get me, I knocked her off as soon as I saw her. Still, it hurt- but it wasn't anything like the pain I have read described.
I remember being really defensive against wasps and trying not to get stung, then one night (I had been drinking heavily I will admit) when I was out back of my house I noticed a new nest near my front door and started spraying them- I got stung and was shocked that it wasn't much of anything at all. Granted, I was fucking tanked at the time. Still.
Not to diminish the pain you felt Guts, it sounds awful.