Is this a good idea?

I've been consistently going back to old beast tribe dailies to work on my collection of anima weapons, and I notice I'm accumulating more grade IV materia than I'm ever really going to meld.

I was thinking instead of transmuting these with Mutamix to see if I can't get either the equivalent level crafting/gathering materia or possibly even grade Vs that I actually COULD use. But will this work out the way I'm thinking or will I end up with lower materia grades as well? It'll be only grade IV materia that I'm transmuting.

I did that. And mostly I got T4 materia, maybe 1 in 10 or 1 in 15 got me a T5 materia.

As long as the ONLY materia you throw at Mutamix are T4, you will not get anything lower than T4 back. If you start tossing some of the T5 that he gives you at him, they increase the chances of getting a T5 back, but 4 T4 and 1 T5 don't give very good odds on getting a T5 back. Better than 5 T4's, but by no means good odds.

One thing to note however is that when you transmute materia via Mutamix, you will not receive the same materia that you've added to the mix. For example, if I throw in a Piety Materia IV, Mutamix won't give me back a Piety IV as a finished materia.

It's worth keeping in mind if you were considering recycling the substat materia like Accuracy or Determination because they're most likely to be more valuable to sell on the marketboard.