Letting ChakraCore run means effectively "wrapping Chakra to emulate V8's APIs". There was an experiment to do this for SpiderMonkey at one point but it's a lot of work to maintain.
Here's Mozilla's effort to implement the V8 api on spidermonkey if anyone's curious:
The Node fork JXCore supports Chakra, SpiderMonkey and V8 right now. It's pretty fast, probably because it's based on a pre-0.12 node fork when node was known to be faster than it is presently.
I'd be curious about Nitro/JavaScriptCore (in WebKit). For certain workloads (haste-compiler JS output from compiling Haskell) it performs much better than Chrome or Firefox for me.