I have recently been running 1/3 of my runes as CD reduction. Getting Haunting Visage as a first item helps, and then grab the golem whenever possible (Gankplank's actually a decent jungler with Golem buff). You can solo golem with ease at about lvl 8, and lizard at lvl 9, probably a bit earlier if you want, but you don't want to drop too much health.
Also, getting Sorc Boots will help the damage on both Parley and Cannons. Good to get, although I tend to only get boots late in the build since Gangplank is so fast. I prolly should be getting boots earlier though.