We shall just have to see. We have yet to see any problems with LZ4, even on the lower-powered CPUs. Also, as you yourself like to say, you have to build a FreeNAS system with at least reasonably capable hardware to see good results. We also didn't prevent someone from turning it *off* if their systems are just too weak, but turning it on by default means that the majority of folks will be able to benefit from a feature they probably didn't even know existed, and we design for the majority, not the minority.
I will also point out that the compression algorithm is pretty smart and "short-circuits" itself out of the way if it's not able to be effective with certain kinds of data. It's more like saying "if you can compress, use lz4, otherwise don't."