Performance per watt is pretty good on ARM. Most performance these days is about IO. How fast is the memory subsystem?
There are intel chips with 400 gigs per second memory subsystems. But the cores run at around 1.4ghz. Even a single thread with that much memory bandwidth can be much faster. Likewise with cache. If you have 32MB of cache, you can do quite a lot more processing in cache, and be lots faster for many things.
Considering that macbookpro laptops haven't really gotten better single thread performance in years - the things that really matter are IO, and performance within the power/heat budget.
ARM is competitive now within the macbook power requirements. The A10 chip is 75% as fast as the i7-6600U in some single core benchmarks. At a much lower power budget.
Within the chromebook space, where ARM and Intel laptops are currently competing - Intel often produces faster laptops. However, they are also often more expensive. Lots of people in many-many reviews say their ARM based chomebooks are too slow.
Having been through the raspberrypi 1,2,3 performance leaps, I can say that the latest one is quite usable as a desktop machine for word processing, web browsing and media playing.
The Samsung Chromebook plus with ARM is shipping, and the one with Intel isn't yet. The ARM one is cheaper, and the intel one is apparently more performant. Both have a similar battery life. The ARM one has better support of android apps. GUI app development is now 10x more active on ARM compared to intel, and the apps are optimized more for ARM.
GPU performance is improving faster on mobile chipsets than Intel is improving. Because of the demand from mobile VR applications. Some are predicting performance will be faster than laptops in a mobile power budget this year (see ARM Mali-G71). It's even faster than some nvidia discrete mobile chipsets. Since many apps are performance limited by the GPU, and not the CPU, I'd say ARM chipsets will win for many users low-mid users this year.
ARM is already taking over the low end laptop market. I won't be at all surprised if they take the mid range market by the end of the year. Especially as the market for chromebooks, and the low end laptops picks up. Because then more laptop class ARM based chips will go into production.
I think the demand will be there, as the android using market also want to use laptops that sync better with their apps.
[ED: add note about high performance ARM chips coming]
High performance ARM chips are coming to super computers. The Post-K super computer by Fujitsu is expected to be one of the fastest computers in the entire world. If not the fastest. Will ARM take the super computer performance crown? I think so.