Is it sufficient?

Hi, could you define "block" for us? if it's merely covering the fans, since the heatsink is designed to allow air to pass through, you should still be able to ventilate and effectively cool the heatsink (the heatsink may act as sort of a ventilation pipe/tube sort of thing). I'm not sure what the performance numbers of the scythe ninja 2 are, but honestly 3.2ghz is a tiny overclock for the q9550, achievable with even the stock HSF.

How much further do you want to take it? I wouldn't be afraid to hit 3.6ghz with any half-decent aftermarket cooler, and with the ninja 2, I'm sure 3.8 wouldn't be a dream.

Usually the limit is set by

a) voltage: the higher we raise this, the more stable we're likely to get our q9550 at higher clocks, at the cost of losing efficiency, meaning more energy is being turned into heat rather than processing power. It gets hotter, and sometimes hot enough past the normal thermal specifications of the CPU. I'm not sure what the limits of the c2q series are, but I wouldn't go past ~74 degrees celsius.

b) Motherboard and PSU quality: another factor that determines how effective/stable your voltages are.

Either way, I'm pretty sure 3.6-3.7 will be easy and safe, with 3.9-4.0ghz being your upper limit if you turn on the AC.