This is where I try to remember the days of 2e. How did I make high-level play more challenging for my players?
I MADE S#@$ UP!
Seriously, I know it sounds like too simple a solution to too big a challenge, but sometimes, the guy behind the screen has to have some fun too. Make stuff up, cheat, learn the rules, and then ignore them, fudge die rolls, AND PLAY GOD!
"High level villains are too risky, I don't want a TPK." Okay, when you roll it, soften it.
"High level villains are too soft for my players." Tell your players they didn't roll well enough to beat the DC of the villain's ability.
"The fighter is a broken little toy soldier next to the god-fueled cleric and the pyro wizard." Occasionally, throw the guy a bone, change a stat block to include magic immunity and susceptibility to physical attacks.
"High level encounters are too time consuming." Wing it.
Also, make your players know their characters. Of course, having them play from low (not necessarily 1st, but 5th or below, for sure) to high ensures that they will learn their abilities, especially if they spend enough time at each level, and experience a wide enough variety of tough encounters to have to figure out what to do when party resources run low.
The biggest challenge for me is keeping it fresh. I can do it, but it's tougher with published adventures. Especially if they're keyed maps, and not event-based. But, it's not insurmountable. I just default to 'make s@#$ up'.
Of course, I still want Paizo to publish their take on high-level play, so that I will be able to make LESS s@#$ up. :-)