The reason prep time is increaced is-
1)I was a huge just wing it guy and really dont think that belongs in 4e
2)when I say this group is min-maxed-I mean it.,
I just wanted to say that for #1, there's no reason you can't wing it in 4E: I consider it to be one the easiest and most balanced editions to do that with.
What do I mean? Well, between the notes on how monsters are created and the improve rules on 42 in the DMG you can have an excellent idea of what your attacks, Damage, defenses and HP should be. The rest is your creativity!
On the second issue, if a character is really min-maxed, I'd talk about it with them...it's possible that one or more characters are using some "grey areas" in the rules or just aren't doing things legally. If they are all legit, I'd take some time and talk about it, because shifting systems is no way to solve a rules min-max problem...it will just follow you into other systems. You might not think so, but many rules light systems can exacerbate this problem by leaving the GM with no "rules" to back up a decision as to why something can't be done. That can cause players with strong personalities to really bully their way into getting what they want.
If you're burned out on 4E, that's one thing, and as has been suggested, I'd say taking a break is in order. Thinking that another rules set is a magic bullet that will solve these problems, however, is likely to leave you frustrated.
--Steve