The radius is in part to allow the top to move as moisture swells/shrinks it. If there's no room for it to move, I'd say there's a greater chance it will crack one of these winters. If you haven't glued it to the rim yet, redo. You also need the height the arch gives so that the neck pitch will be correct.
I used to build my guitars with flat braces. A lot of people do as well. I built a 000 for my sister 27 years ago with a flat top. It has not sagged, cracked, or needed an action adjustment in that time.
So- re-do it if you feel the urge, but I don't see it as imperative. JMHO