I would guess that if you did a full stress/strain analysis on the QA1 vs. a fully seam welded and gusseted stock K you'd find loading situations for both where each one outperformed the other. I don't think either one is slam dunk "stronger" overall than the other. The fully welded and reinforced stock K would be more durable just because of it's construction, more likely to deform than break anything. Whereas the tubular K would likely fail at a joint. But that failure would probably take place under a higher load than what would deform the stocker. Regardless, I don't think you'd notice the difference on the street, or even at the track. But that's just my guess, and we all know what assumptions do.
For my money though I'll keep seam welding and gusseting my stock K's.
