It is always good to provide the latest PHP version to customers as fast as possible. Gotta be top of the notch. Newer PHP versions are usually more secure, faster to execute code, and enforce cleaner ways of coding PHP scripts. And from a marketing point of view, it's great to provide latest versions for software as it is obsolete quite fast.
Funny because I count around 30% using the numbers on the provided link.
Btw, why "only" 20%? 20% is a whole lot already. 20% less time to execute scripts = faster page loading time for visitors, better retention, better SEO, better energy consumption, 20% more websites hosted (or visitors) on a single server...
8.3 doesn't seem to be faster though, but I'll still push it once WordPress becomes compatible.