Multi-Materia Melding Success?

[Raikki]

My cuirass has Defense +12 (Bloodwall III) and Vitality +13 (Vitality III). The success rate was listed at 25.50%. I checked the success rate of adding a Vitality +17 (tier IV) and it came up 9.60%. Success rate depends on the rolls and tier of materia with lower success rate the better your materia is, so your chance of getting two max roll Tier IV on one piece is very poor (I think around 15% but haven't tested max rolls specifically).

I know the system is designed to appeal to the hardcore crafters...but would anyone actually be interested in the gil/time investment required for an entire set of M5?

Honestly M5 is out of the picture now. I think realistically we're looking at M3 for hardcore players. It's going to be an absolutely enormous investment getting M3 with Tier IVs on all slots and it's much more productive to work toward that than spend all your resources trying to get a fourth materia on one slot.