Howdy John1911 and et al.,
I've got a G20 that is several years old. It doesn't quite get shot as much as the other Glock patterns in other calibers. The 10 mm round is a fun round but it is pricier than other rounds both for factory loads and reload components. That is one of the principle limiting factors why this pistol doesn't get shot as much.
However, I'll clarify that mine has over 5K + successfully launched through it. This has been roughly an equal mix of full charge 10mm rounds and 40 SW factory ( I use a conversion barrel, I call it my G20/40 setup). No issues with the pistol in running through good quality ammo. It even runs the sub-sonic LEO Winchester Silver tip 10mm ammo well too.
Using the full charge 10mm ammo busting on the 300 meter rams are not too difficult once you get dialed in. One of the range masters was so curious to what that there pistol was. I told him that it was a Glock 20 in 10mm. Never heard of that. So I show him one of the rounds compared to a 40 SW which he uses. Wow, can I try that? She barks pretty good! He wanted to know right then and there if there was a conversion barrel for his XD to shoot 10mm. Sorry it doesn't work that well going the other direction, I'm not even sure it is possible with the XD.
My pistol has represented the same extremely satisfactory quality and consistency as all of my other Glock patterns. They run well! The changes I've made to my Glocks have been elective to enhance my shooting experience, not to correct a function deficiency. I've changed out the factory front sight to a thin tritium sight to match all the others. I like having some daylight on both sides of the front post and the tritium is cause I use my pistols for fun and serious applications. I did put a factory adjustable rear sight in anticipation of needing adjustments to accommodate the conversion barrel, but it turned out that both barrels shot to the same point as long as things stay under 30 meters. The 40SW just doesn't have the gas to keep up with the 10mm past that point.
The only other modification has been to install a Glock OE #3.5 connector to make the trigger more manageable. That will probably change to one of the Lone Wolf Distributors #3.5 connectors shortly. If you've not tried one of those yet, it is well worth the small investment to vastly improve the trigger manipulation.
So there you have it. Mine has proven to be very reliable. So much so that it is my trail gun of choice. Jaunts about SE AZ will always find the G20 with me. Using the G20/40 configuration allows me to run the beast in limited/major class at the regional USPSA matches using the less costly 40SW ammo.
You last question is one that I'm not sure there is an easy answer to. I believe that there is a 10mm Witness. I've only shot one for a couple of rounds. One of the gun store owners was out at the range the same time I was and he brought one out to try. He allowed me to run a few through. I believe the Witness was a bit less expensive. It shot ok for those few rounds. It was somewhat close to regulated with the sights out of the package, but no where like other pistols which were good to go from the box. The trigger was very interestingly different from a Glock trigger, livable but needing attention. A good friend also has one of the Dan Wesson 1911 in 10mm. His was one of the first patterns run by DW before CZ purchased them. The craftsmanship on this one is pretty good. It is not in the same price range as the Glock, so be prepared. It shot reasonably well. It is very selective on ammo, it did not like some of the hollow point ammo versions. Also finding magazines could be challenging depending where you are.
I hope this information has been of assistance. Go out and try a few patterns before you buy one. You can also check out some of the other forums for more information (ie. 10mmtalk) Enjoy the testing!
Cheers!
