Grade IX CP materia is just as good as grade V/VII.

Just a reminder: the chance to successfully overmeld grade V materia already bottoms out at 17%/10%/7%/5%. It doesn't go any lower.

If you are buying your materia on the market and Craftsman's Cunning Materia IX are cheaper than Cunning V/VII (on my server, Cunning IX materia have been hovering around 50%-75% the price of the others), just use those in your new jewelry.

I've been selling off my stockpile of Cunning V's and VII's and buying IX's off the MB. Easily more than doubled my supply at no loss. Cunning V's in particular spike to ridiculous prices, enough that it's worth spending scrips on them, selling them, then buying all 3 type of XI's instead of scrip farming them directly.