As I understand it Apple's Core Audio is pretty solid. Nevertheless, used Focusrite Scarletts employ the same drivers as new ones, are unified across the line, and the 2nd gen units are relatively inexpensive. I use a 2nd gen 18i20 and Octopre Dynamic for work (not very demanding playback and occasional recording).
Having never used the Helix interface I'm surprised the latency is so bad. Have you done a loop back test?