Not really parsing what you're saying here. Doesn't matter though...I'm out for the night.
If Declarations are viewed as problematically 'too easy', and arbitrary judgment calls on the behalf of the GM are themselves considered to be a problem (arbitrary action being deemed 'd1ck-ish'), then those arbitrary judgment calls are not a viable solution to the original problem.
Implementing a 'soft cap' houserule, on the other hand, if accepted by the table as solving the problem and as not being the source of a problem in its own right, WOULD be a viable solution.
Err, no. Not at all. Unless you call stating NPCs rule 0 - that's how the GM kills characters - encounters.
(exaggerated for emphasis)
Player of recently dead PC: 'So, what were the stats on that mortal beat cop who just one-shotted me?'
GM: 'Roughly Legendary in anything physical, with stats elsewhere varying from Good to Superb, why?'
Player: 'Just wanted to know why it is that a mortal beat cop could compete in the Nevernever Olympics with a reasonable expectation of making it to the finals...'
GM: 'Because I said so, and I'm the GM, and what the GM says, goes.'
Player: 'And so do his players.'
Yes, statting NPCs with the express intent of killing the PCs is an exercise of Rule 0.
All arbitrary decision making powers placed in the hands of the GM are an exercise of the principle of Rule 0.