Optimized characters aren't going to be your foremost challenge, Typhina: it's the fact that you have veteran players that really needs to be addressed. A wizard who's really exploiting transmute rock to mud or even the humble gaseous form can chop entire levels off a dungeon. Smart players running non-spellcasters are often motivated by their lack of spell options to really think through tactics and dirty tricks. "They aren't on a siege footing, so we'll just hang around out here and kill patrols - thin out their numbers. They don't have a cleric? OK, bring me that cage of plague-infested rats and I'll sneak them into the place."
Pre-written modules tend to assume that everybody in the dungeon minds their own business and, except for a few scripted occasions, will not leave their areas. But in many ways your players will feel a greater challenge if you fully exploit the enemy's intelligence, position and resources with flexibility and thorough nastiness. (If the opposition is led by a being of superhuman intelligence, like high-level wizards or aboleth, you should also metagame slightly by giving them a real knack for guessing the PCs' tactics and weaknesses.)
Modules that would ordinarily be a breeze can become challenging again if you assume the enemy prepared the ground and established defensive tactics that are stronger than those presented in the module. If the PCs try to transmute rock to mud to get into an isolated area without going through the guards, they discover lead lining/guardian spectres/hollows filled with explosive gas. Guard patrols should be numerous, erratically timed, required to exchange passwords, and equipped with signaling devices; and everybody in earshot, guard or otherwise, should respond to those signaling devices as soon as possible (except a few sentries left behind in case this is a feint). "Panic buttons" like one-time summons and necklaces of fireballs should be distributed intelligently and employed at the first sign that the PCs represent real danger.
There's a lot more of such advice on other threads. I guess I could have summed it up with: Beefier bosses are good, but smarter minions are just as good and can allow you to use modules that you might initially have dismissed as unworthy.
