I own 3 TM G26A's and having said that I haven't had a problem with them so far. Running propane with a few drops of silicone oil to keep the gun and the o-rings in the mags well lubed goes a long way. I've taken the fill valves out, soaked them in silly oil for 24 hours, popped them back in and they've held gas ever since. No problems with stock TM mags.
Here's a quote from a retailer that I got :
My suggestion, is to go for the Marui mags - not only for compatibility reasons, but simply because they are more durable and should perform better. In the long run, it's a wiser financial decision. Anyways, let me know when you figure that out.
Having said all of this I just recently acquired a KJW G27 and it's nice with the metal slide and outer barrel but the mag leaks even after I've soaked the o-ring in oil. Also the heavier slide and outer barrel tends to use allot more gas than the plastic one.
It's also already been proven by Mike that the KJW might shoot a bit harder but the TM will out range and out last it all the time. Forget everything you think you know about FPS and continue reading. Heck, even I've made mistakes and have had to go back to reading the forums.
You see, what everyone is telling you on this forum is true. And I have, as well as others have, first hand knowledge. Listen to what's been said to you, and stop being that person that asks for advice only to be upset when people don't validate what you've already pre-determined to do.