I was hitting walls at ~:30. This is much more difficult than w/fins...my time underwater was only an extra 33%, but my resting time was like 300% greater (there is some room to cut that down I think).
As far as "pushing limits"...remember that there's really an entirely different dynamic between w/fins and w/o fins. With fins is all legs...w/o fins brings upper body into play. You're not nearly as hydrodynamic w/o fins. I would say that you're pushing your limits a lot more w/o fins. Just ask yourself this...which is more difficult. OK then...that's where you're pushing your limits. Yes, you could "push you limits" either way...but where is the most room for improvement, and which method would give you the most crossover benefit?
Let's say you really tried to go fast with fins...do you think that would "transfer" to without fins? I don't. On the other hand, working the no-fin method and getting good at it, I think you'd end up smoking when you strapped on fins.
Hypoxics by their very nature are not supposed to be aerobic exercise. So speed, elevated HR, etc. are not really what you're after. What you're working on is ability to work with less O2, to stay calm and controlled, etc. I think those are better accomplished without the fins (where you're underwater longer and having to work harder).
The semi-sick thing is that my best 50 Free (short course yards) times are in the low 20s (best ever :20.1). Comparatively speaking, I'm already finished and out of the pool before finishing my first underwater.
OK E...so step up and let me see what ya got.