I only noticed that developping a SR weapon to lvl 10 gives it another 25%, which makes it a total of 50% when used as material SLVL on my SSR weapon SLVL7 Atk (++). So going from that 1 SR weapon lvl 30 can get my SSR weapon SLVL7 to SLVL8.
Oh gawd if only I know that much sooner...