Anyone have experience with this aperature rear sight?

AK peep sight

www.krebscustom.com/CustomParts.html

How does the aperature size compare to the MoJo sight?

I beleive the apeture size is larger than that on the mojo. But the Krebs peep sight has the built in range adjustment while the mojo does not. On the Mojo, you sight it in at a specific range and set it there.

I have only seen Mojos on bolt guns. I have used the Krebs on an another's AK. I do know that the outer portion of the Krebs falls in line well with the front sight hoop, so it is very natural to keep the front post centered.

If you can use different sight pictures (above the post, flat tire, covered by the post) you can adjust your aim at varied distances easily without moving the rear aperture.

RS39,
The Krebs sight seems like it would be better on an AK due to the reason you mention, sight hoop alignment.

Other advantages of the Krebs appear to be the meter graduations like the original rear sight, the ability to zero at the 100 setting then switch to 'P', battlesight zero, and an original look.
I suppose one probally wouldn't have time to, or want to, mess around with rear sight settings in a fast paced competition or fighting situation.
I think I may have sold myself on the Krebs.  

Quoted: Other advantages of the Krebs appear to be the meter graduations like the original rear sight, the ability to zero at the 100 setting then switch to 'P', battlesight zero, and an original look. I suppose one probally wouldn't have time to, or want to, mess around with rear sight settings in a fast paced competition or fighting situation.

That is exactly why I prefer the Krebs over the Mojo.  Don't get me wrong.  I like my Mojo on my SKS, but the Krebs is a better battle sight.

Krebs says their opening is 3.9mm.
The MoJo and Williams openings are 0.150".
3.9mm = 0.15354".
Close. Another reason toget the Krebs version.