It sounds as though the only issue you're having is with green side bunkers so I think that it's probably pretty safe to say that that the grind/bounce isn't incorrect for your swing. I especially wouldn't suggest changing because you say you're gaps are better and you have distance control. I play a 58 degree M grind or L grind and I will switch to either have more or less bounce to accommodate how firm the course/bunkers are. Anyhow, in my experience I haven't had any issues with the 58 degree M grind from green side bunkers. I imagine you're correct and that because you had been using a 60 degree you're apprehensive and not accelerating down through the sand to "blast" the ball out. You also may be over thinking the shot and trying to compensate somehow. Since you're catching the ball thin my guess would be that you are trying to help lift the ball from the bunker. Typically when someone is trying to help the ball in the air they are doing two things:

1) too much wrist to try and flick the ball up in the air

2) keeping too much weight on their back foot and not moving through the shot

If you were confident in your ability to play from a green side bunker before putting the SM5 wedges in your bag it will only take a little practice. The M grind is very versatile, the sole has a substantial amount of camber that along with the bounce will keep from digging and heel relief that allows you to open the club face. I suppose you already know this but my point is that there isn't a design flaw in the club. Once you realize you don't need the 60 degree wedge to create a nice high bunker shot that will spin and land softly you will have confidence again to make those important sand saves.  

Thanks for your very helpful post, Jonathan. I am absolutely confident in the design and performance of the Vokey's - none better! This issue is user, not the arrow!

The mechanical flaws you mentioned are my top two to check and work on next time I can practice. We have had now 9 straight days of rain or conditions that were too wet to open my home course. Hopefully I can get out soon to practice and really focus on one or two positive swing thoughts and swing away. Thanks for your help and encouragement!

Have you tried one of your old wedges alongside as a comparison?  The other thing is that the bunkers tend to get pretty thin this time of year after being worn down from a summer of golfers hacking away in them.  Don't discount the sand as possible source of woes as well.

Maybe get a quick lesson from your local pro.  If you're swinging tentatively as you say, maybe there is also a swing flaw that crept in because of it.  Make a mental checklist of the fundamentals of a good bunker shot, focus on those, and swing away.  With awesome wedges like that in your bag, you should be knocking close in no time.

Great thoughts, Keith. Thanks for sharing. I did think about trying my old 56 and 60 alongside my new 54 and 58 just to see what would happen. But, the sun disappeared and the rain bullets are dominating right now. Hopefully we'll get some decent weather soon and I can get out and practice.

Also, you're so right on having a clear thought process. During my last round, I felt like I had 1000 things going through my mind in both of my green side bunker shots. Simplify, focus, and put a swing on it!