It sounds like they added it to the list because other people were reading too much into the omission.

> Microsoft doesn’t even mention it on status.modern.ie, however, so it isn’t likely to support it anytime soon.

http://techcrunch.com/2014/07/22/mozillas-asm-js-technology-...

> Some folks are incorrectly thinking that since it isn't on our list that we're not aware of it.

https://github.com/InternetExplorer/Status.IE/issues/116

I don't think "under consideration" means anything beyond that they're aware that it exists, and maybe that they aren't categorically opposed to ever implementing it.

ASM.js is all about backwards compatibility. The level of speed increase is entirely implementation defined. They could add even the most basic level of support/speed-boost for asm.js code and technically support it.