Usually, Oleg recommends to avoid doing that unless you ensured better heat dissipation for the CPU, i.e. attach a heatsink to it.
My wl500gp runs @300Mhz for several months now, but some guys were not able to even boot @300Mhz (i.e. bricked the box, but recovered using a console), but clkfreq=288,144 worked fine for them. My other wl500gp with 128mb of ram was unstable @300Mhz (i.e. showed memory errors during tests), it's running @264Mhz now.
And yes, I've a heatsink attached.
IO performance does not change, as PCI is still running @33Mhz.
I don't think the 30 mhz will make such a big difference. At least, not as much as I would like to see for the pain it costs to recover it. 300 mhz would be nice though, but the probable need for a heatsink makes it less interesting.