The feat itself is not overpowered. By the time you can use it with fireball, you could also be casting cone of cold. That means either 9d6 in a huge area with a reflex save for half, or 9d6 in a smaller area, with no save. At higher levels, your cone of cold just keeps getting bigger and doing more dice, whereas the fireball maxes out at 10d6.
When you can use it with cone of cold, you could also be casting delayed blast fireball (in which case hide spell is better) or are 1-2 levels away from Horrid Wilting, in which case hide spell stinks.
Also, you have to look at everything that ANY company puts out with a grain of salt, not just S&S. Their heat generating magic is setting specific, and has reasons for being there. It is not just an excuse to get drow chicks in bikinis (especially considering their setting doesn't even have drow).
I personally allow Hide Spell as written in my campaign, but no one has wanted to take it yet.
[EDIT]
Also, increasing the requirement to +3 means you have to compare it to maximize spell. With maximized fireball, you do 60 points, save for 30. Hidden Fireball would do (on average) 35, no save. I think the maximise wins out every time.
[/EDIT]