The Shadow Dance change is a bugfix, when they made her dash a strikethrough, it made it travel slower for some reason. Now since they have been made aware of it, Riot now finally have the time to fix it in the upcoming 6.3 patch when the start of the season has calmed down.
Basically:
1600 missile speed old dash is not the same for a strikethrough dash. It needs to be 27% more for the strikethrough dash to match the old dash with no strikethrough. So 1600 -> 2000 is a 25% increase which is close enough.
More details here: http://boards.na.leagueoflegends.com/en/c/bug-report/ErzYFuxe-gameplay-akalis-dash-travels-slower
TLDR: Akali's dash on the current 6.3 PBE has always travelled that fast. It just has been travelling slower since Patch 5.9 onwards.