Well technically it would be since clock speeds can be altered via software.
This processor is designed for low power consumption not raw performance. There is probably a little bit of head room left but I would expect not much. Apple designed this to keep power consumption to a minimum not maximize performance. There are physical limits imposed by the process technology and design techniques used.
The next core will very likely be A15 based. You will see 4 cores running at lower clock rates than Apple is using today.