I have been able to make a compact full adder with a carry propagation of 2 ticks. This design still only uses torches and dust and remains within a 6 by 2 by 8 form factor, giving it a bounding box volume of 96.

I have made some changes from the previous design due to slow carry propagation and sum. The main change that made this possible was removing the final torch from my XOR gate, which converted it into an XNOR gate. For the carry, the NOR of A and B (this is already part of the XNOR gate) is then directly combined with the inverted carry-in in a NOR gate (carry NOR). This step is where the two-tick carry propagation occurs. Then the last component of the carry comes from the XNOR gate, which has already computed the AND of both A and B inputs; this is then directly ORed with the carry NOR output.

For the sum, a second XNOR gate is used. Wired with the output from the first XNOR and the carry input (which had to be un-inverted). This gives a sum speed of 4 ticks.

I have been able to tile this adder with a one-block gap between each adder. I have uploaded a schematic file for this on my MineMev(RUBENHILLIER).