There is no real slow cast times on Red Mage if one knows what they are doing. You know dual casting, your next spell after Jolt is quick and even Jolt is not considered slow... If you want to know what slow cast times are try blm.
As for nerfing i agree 100% that Red Mage should not be nerfed just because it is more powerful then other jobs, instead they should buff the other casters being smn and blm and they do need it