I contacted Phill to ask a similar question. To paraphrase he said 'we don't quote sensitivity as we don't feel it's useful'.
I took that as code for 'not very efficient', nonetheless I bought a C4. It sounds great, and is not very efficient.
I have used it at an 'cajon' jam with an Elf and it was fine. Gigged it once, up on a stool it worked as a monitor but I leaned heavily on the PA for foh in a venue where I would not normally need any pa support at all.