I actually tried an extension similar to the straight on a Kahr PM9, that was made by Pearce. It was made of plastic and it worked OK, but I didn't feel that it did that much for me. Part of that is probably because I'm just used to using pistols as they come out of the box. I also started as a Bullseye shooter shooting .45ACP back in the mid 60's fresh out of High School and competed in both Hardball and Centerfire phases of the National Match courses on a regular basis. I was taught then to grip a 1911 with the 2nd and 3rd fingers, and let my pinky just relax and my thumb either relaxed or resting on the thumb safety. So it's very common for me to grip any handgun with just two fingers of my main grip hand. Since I taught at a State LE academy for some years I tend to force my grip hand into my support hand straight and overlap my thumb almost like a golfer. Then the hands push/pull together and it gives pretty good control. A small grip like the Kahr and the Rohrbaugh are a challenge but not unmanagable.
I like the pistols because they are so small and when I think of giving up something in size to get a better grip then I start wondering if I should be carrying my Colt Defender instead. But that is just me and doesn't apply to anyone else other than as "food for thought."
As with many things about shooting, if one thinks they help, then they probably do.
