I've built a new design for the fastest possible insta-carry adder (3.5 ticks for 8 bits and 1 tick for every 8 bits after that). I used glowstone to make some interesting XNORs that allowed me to put the carry signal directly on the carry line and take it directly off, a concept that was pioneered by anomalouscobra when he built the first adder that reached this speed. This design is 2-wide tileable (in pairs), 12 long, and 10 tall.