I bought a ghost ring sight for my Ruger Mk 1 several years ago and have fallen in love with it since. Improve my groups? Absolutely. Putting the target in the center of the ring and keeping it there is much easier than aligning a notch-and-blade setup and then trying to hold that onto a target, especially an irregular one.
People who have tried my gun like the sights as well and want to know where to get one. The original source seems to be deceased so I had some made.
The originals were also made of soft steel which tended to bend if the gun was bounced about, say by TSA agents with no concept of the phrase "This Side Up". Mine are 4130 and laser cut so should be more durable. I'll have these on the market as soon as I can figure out how without incurring the anti-gun wrath of e-bay or Pay Pal. The current batch fits Rugers with adjustable sights. I'm not sure about others. If the demand is there, variations can certainly be produced.
More as it develops.