I have taken my SR1 to 180' (55m), and even allowing for narcosis, the breathing resistance is absolutely non-existent. I really believe them when they say it has the lowest WOB of any market regulator in the world.
There are plenty of regs that deliver far more gas than any diver could ever possibly breathe at depths far exceeding recreational, and even common technical O.C. limits. WOB numbers on any one of a number of high end regs are so low that comparison is just not that useful.
I find the quality control problems with the SR1 ample reason to avoid them, as there are many other excellent choices. By far the most important quality in any regulator is reliability, and practically all regs are very reliable these days. Why choose one with known problems?
Regarding inhalation resistance, all regs are limited in cracking effort by case fault geometry, in which pressure differential between the diaphragm and the exhaust valve will cause freeflow if the reg is adjusted to crack too lightly. Poseidons skirt this problem by using a side exhaust valve, and the SP D series (D300/350/400) do so by using a coaxial diaphragm/exhaust valve. which almost eliminates pressure differences between diaphragm and exhaust valve.
Most high flow regs make up for limitations in cracking pressure by increasing venturi assist, to the point at which there can be positive pressure during inhalation; that's sort of a controlled freeflow. I personally don't like that feeling of air being pushed down my throat. This is the beauty of the D series. They can be stable at about .5" water (not all of mine can, I suspect it has to do with tiny imperfections in the orifice and occasional seat issues) and have a pretty subtle venturi effect. This makes them super natural breathing in lots of positions.
I have to say I'm disappointed that SP has not reissued the D series like they did the G250 and (sort of) Balanced/Adjustable. I have a feeling that's for two reasons; one is that the regs are different to work on, and many LDS techs are challenged enough as it is. The other relates to what I wrote earlier, there are many excellent choices now and WOB numbers are already far lower than is necessary, or even noticeable for essentially all current scuba situations.
I just like them because I'm a geek and the refinement of the mechanical design appeals to that side of me. (They go forever between rebuild, too) There are lots of dives where I'll switch to my unbalanced (gawk!) 109 alternate, even though the WOB is noticeably higher. I do this when my mouth is dry, because the nice thing about the metal case 2nd is the lack of drymouth.