This is an incredibly simple mod, changing only a single game setting. fMagicArmorPenaltyMin is changed from .05 to 0, while maintaining fMagicArmorPenaltyMax = .30. There were other mods about spell effectiveness, but they seemed to remove the scaling entirely by setting both to 0. This maintains the mechanic while allowing full spell effectiveness with training.

I'm not sure what the exact formula for the scaling is, but you will reach 100% at the same time as you would normally reach 95%, around 50 or so.

Nexus Mod Manager works for installation, but if you would rather not it is easy to handle. Just extract and place the esp file in your Oblivion/Data directory and activate from the Oblivion Launcher's Data Files menu.