Someone, somewhere probably is making them for the more popular LG phones. I would imagine it would be based on the devices popularity.
Yes. Its is just as quick as it was with my G1. The downside to it is when the battery on the keyboard starts to drain you'll start to have a slower reaction and or will need to press the button twice. But with a good battery its pretty snappy. The battery easily lasts me all day, though if i were a heavy texter im not sure how long it would last. A full charge on the keyboard has lasted me up to 3 days...the bulk is there for sure and after a few months/weeks with the super slim S4 by itself you will notice the extra size. It's nothing crazy enough to make me stop using it.