I agree that positioning is critical in defending, but I will say it's a timing issue more than acceleration related.I've already answered this. You have to know what speed training he's had, to know... if he's had none, or little, yes, he can improve.. but to what degree varies from person to person. We are dealing with pretty small margins of improvement needed though. If he's already trained his max, then no... but we don't know what training he does, to really know to what degree he can improve.
It's not the speed, as much as using explosive power the first few steps for acceleration... (which incidentally, would help top speed some too)... But acceleration improves positioning, which is ultra critical in defending.
If he learns how to adapt his speed, or lack there of, he will learn how to defend at the NHL level. I present Justin Holl as proof. It took him a bit but when he figured out he needed to be in place before a certain time, he became good enough for a top 4 role