I think 20% is more likely than 80% for raw compute tasks - ARMs are competitive with intel in performance/watt, but are not much better at it (and definitely are not competitive yet in absolute performance). People are going to buy it for the 20-hour battery, and then run the CPU at 100% to run emulation which will eat the battery in 3 hours.

However, most people run a small selection of programs most of the time (web browser, word, excel, outlook), which will likely have an ARM native version sooner than later, and the long tail of programs they must have are often not compute intensive, and spend 85% of their time in the OS doing graphics/io which are native. So overall, even if the CPU emulation is 20% speed, the overall feel is likely going to be 80% or better for most people.

Gamers shouldn't hold their breath though.