My idc;s with the sz49 and modded stocker 810's never go above 74, which seems about 620.

would an injector with a smaller scaling number such as 640's or 720's drive better around town and idle better than modded stockers at 810?

if you're getting bad idle and low thorttle from poorly modded injectors then a proper spraying may yes...

but you don't want to exceed 80% idc's, so you're right where you should be... you don't want to run 620's static...