I don't think I'd mind a minimum investment of levels in God for that particular benefit, though I'm not sure how it fits in with everything else. There certainly are a lot of abilities that suddenly become amazing at 20 HD, even if they were pretty darn good as it was before. At the same time, this is a class that requires a lot of DM attention, making sure you're acting within the bounds of your portfolio, making sure you don't lose your worshipers, etc. However, that can't certainly be handwaived if the DM just doesn't care, or is lazy.
Maybe the limit of spells that can be cast "all day long" can raise depending on your number of God levels in comparison with other levels.