I have experience with 25 Point builds and powergaming in general. The probably craziest thing I let them do was a Sigil campaign where we played with Pathfinder as Base rules and included all the 3.5 books we had to our disposal (which where actually allmost all of them).
I wrote this to see if someone else had experience with evil campaigns. One of my biggest concerns is actually the following campaign.
They are going to start at 9th level and will get up to 2 mythic Tiers (as a counterweight to the CR adjusment of the evil group).
The bad guys will advance till level 16 and I gave them a common goal. I am using the Golarion backround and Cheliax helped them to escape an angelic prison in the first adventure, so that they could help with an rising revolution in Isger which is backed by powerfull Individuals.
One of my biggest concerns with the bad guys is our Antipaladin. He´s also our youngest player (somewhere in his early twenties while the rest of us is over 30) and while the group knows each other quite well (we are playing pathfinder for 5 years at least now) he seems a little eager to wreak havoc. Also he is unbeliveable tough (we are talking 300+ Hp, Ac 40 and saving throws above +16). All the other characters are either lawful or neutral evil which is way easier to handle than chaotic evil in my opinion.
I already told them that I will add little changes to those bad guys as soon as they go to NPC status but I think it wouldnt be fair if I made them considerable less effectiv as they build them.