Remember that spells like Evard's and Solid Fog basically require magic to overcome (freedom of movement/gust of wind) and that spells like Divine Power serve to tip the balance in favor of magic users. Even spells like Improved Invisibility can cause severe issues, while 3rd level spells generally do not have this problem.
Most of us are operating on a "lean upwards" mentality, where we create feats that allow 1/day casting of non-broken 4th-level spells, like restoration, wall of fire, etc. Obviously, you can do what you want, but it seems like an 8th level cap would massively increase the workload for no real payoff.