On
this page it says max speed is 628 km/h (check "email message 1" near the bottom of the page). Looks convincing to me. 678 km/h might be a typo (7 = 2). I've seen some pages claiming 678 km/h to have been achieved at 2 km altitude! So it might be a wrong number or a theoretical estimation. Otherwise the maker of the page put in the best numbers he found I guess.
In message 2 there's a part that says:"628 Kph with FIAT engine and more than 650 Kph with DB engine" for the Re.2005 "LW". How accurate this numbers are I don't know. However, if there was improvement in the change from the Italian to the German version then the 678 km/h number seems even less likely to be the correct one. I've just ran through the text tho and might've missed something. I hope it helps.
Cheers,