You can, as the reasons for that limit are heat and amperage. CFLs get a little warm, but nowhere near as warm as incandescents, and LED bulbs are cool, so half of the danger is eliminated. And since voltage is constant and wattage equals voltage times amperage, the lower power draw reduces amp draw and thus won’t burn up the wiring. From a safety standpoint, you’ve got a lot of leeway and can go up a few notches.
That said, I find the lumen/watt ratio in CFLs and LEDs to be a bit too high, which makes it hard to get just a little brighter. If you go out and get yourself a couple of 12W LED bulbs, you’ll have more lumens than many car’s high-beams, which may be a bit more than your basement needs depending on how big it is. Since 12 isn’t much higher than 9, you may find that the next step up is a bit much while not having any real options in between too dim and too bright.
You could safely put in something like these 54W LED bulbs that are brighter than a 250W halogen worklight, but that doesn’t necessarily mean you’d want to. Just be aware of how much difference in light 1 watt can make with LED/CFL bulbs before going hog-wild and accidentally turning your house into a tanning booth.