My group is much lower level, 6th as you know. And your group is heavy on divination and planar travel, even for their level, and you've got high level psionics to deal with as well. I'm not qualified to give you great advice on this, as the only high level 3E games I've run were various one-shots.
But I've got some ideas:
- First, if you're uncomfortable with the amount of magic items, I'd do an audit. Have them each list every magic item and piece of treasure for you, and check it against the wealth levels for PCs in the DMG. I started doing this when they level up, and it's very helpful. I don't really use it to penalize anyone, but more to make sure that I'm placing enough treasure. If they don't have enough wealth, the CR system gets very skewed, and I also want to make sure that nobody is getting left out. Once you have a good handle on what everyone's got, it's a lot easier to challenge them and not be surprised.
- Remember to have them make saves for items when appropriate. Otherwise, their magic item collection will grow like a cancer.
- Items that have penalties as well as bonuses can balance things out a little. Our rogue has a keen rapier +1, one of the few magic weapons in the group. He loves it, except that it glows brightly and ruins any chance of hiding...
- I like to make magic items with charges like wands and potions plentiful, and permanent magic items rare. And charged items seldom have many charges when found.
- Remember that any economic system has a way of balancing itself. If the adventurers spend a lot of cash, prices will go up, even in Sigil. Bring a chest full of gold into a city, and the value of gold will go down as soon as it's spent.
- If things get out of hand with too much magic, you can always throw something of a higher CR at them if they need a challenge. After all, if your 15th level group is performing like an 18th level group because of their clever gameplay and/or too much magic, they should be ok against a CR18, usually.