I have the custom 5 and the custom custom. To my ears, the custom 5 has a less trebly spike than a 59 in the bridge, but because the custom 5 has a "flatter" sound compared to the other duncan pickups, it may sound a bit scooped and "trebly" as some may say. I have a custom custom in a thinline mahogany strat with a rosewood fingerboard. The fact that my strat is mahogany and not poplar or alder kinda skews things, but even still, in a bright strat, the custom custom is a dream. It is very warm and round. What it lacks in "boomy bass" it makes up for with low mid range punch. It has a very low midrange growl. Palm muted chords have a very percussive click to the sound.
However, in the case of my strat, it's actually too dark and compressed sounding when overdriven. It crunches nicely and the lead bridge tone is to die for, but for heavy riffs, it may seem to get lost in the mix if you're using thick strings and have a dark sounding rig. Chords on the low E string can even get muddy. The clean tone is gorgeous with no ice pick type treble. On a bright strat, this won't really be an issue.
One thing about the custom custom, is that when overdriven, the sound is just enough to hear and cut, but it stays in the background. It stays under the mix without being overpowering, yet is still heard.
I've decided to go with a JB in my strat and hopefully it will brighten my strat up a bit, yet retain the mid range growl.