As an aside, this is SB after all, I took out a D300 yesterday, my umpteenth attempt getting one to not leak. Anyway, it breathed beautifully.
On my second dive I dove a G250 and honestly initially I thought I must have the Venturi on min and adjustment knob screwed in, I didn't. In "felt" harder to breathe in comparison. My G250 functions as it should and cracks at around .9". After about 5 minutes I got used to the way it breathes and it felt fine.
On the third dive I dove a conshelf and in comparison to the G250 it initially felt harder, but again after about 5 mins I got used to it and it felt fine.
My point/s, IMO, a) the D series breathe beautifully, a noticeable notch or three above the others and b) bottom line after a few minutes it seems that most regs feel ok and are perfectly adequate. Unless doing back to back dives using different regs regardless of stated/advertised performance one gets used to them and they all breathe well enough, or even nicely. Brain burn.
I'd prefer a D series for the rest of my diving life but would be happy enough having any of the regs I own as my last.