With proper ordering of the ssyntaxes in 'def-all-ss, yes.

> Do these ssyntaxes currently get optimized away at compile time (i.e. like 'compose does)? Or do they incur a run-time penalty of some sort?

Not yet. The run time penalty should be as small as a function-composition overhead. The optimization away of 'compose is done by the Scheme-side 'ac, so if optimizations for 'orf and 'andf and similar are desired, munging them in the scheme-side is necessary.

Edit: As an aside, the above ssyntaxes are not yet implemented in the ssyntaxes.arc file. For the most popular ones - postfix ?, as well as 'andf && and 'orf //, you can modify:

    (def-all-ss
      //   (orf ...)
      &&   (andf ...)
      #\:  (compose ...)
      ~    (complement R)
      ~    no
      #\.  (...)
      !    (L 'r)
      ?    [isa _ 'L])