Update - I spoke to Aaron & he answered all of my questions.
1. He said if you're a right handed shooter, & shooting suppressed, that you'll get gassed badly with the left hand charging handle set-up.
2. He also stated that an SBR 300 BLK doesn't spue a flame like a 5.56 or 6.8 round would & saw no need for the KX3.
SO .... I told him to build me that upper & got put on the production list. I went with the right charge/right eject receiver. He doesn't charge you until he's ready to ship your item.