As one of the Beta Testers for Ars Magica (Come on guys, capitalize the name) I can reveal that what is planned for the mod, has never in the history of Minecraft Modding been done before. Ars Magica mob spawning rates have been lowered to an extent, and structure spawning rates are now in the config. More spells are planned to come.
As well as Optifine is semi-compatible with Ars Magica, I have been using it since day one and have had no issues with it. I use the Ultra version myself, and other than that the only thing apart from what I've heard other people doing that I do different, is I delete META-INF even though it is not necessarily needed. Try that out.