What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?
According to The GE90 - An Introduction, the GE90 has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350). The CF6 has a mass flow rate of 591 kg/s at take-off.

Exhaust velocity isn't generally quoted, perhaps because it only bears a loose relation to the performance characteristics of the engine. I suppose if you wanted you could find the area of the exhaust and use the density of air to find the velocity.

Looking specifically at the GE90, we see that it has take-off SFC (specific fuel consumption) of 7.9 mg/N-s. Using the formula $I_{sp} = 1/(g_o·\text{SFC})$ given in paragraph 4 of the Wikipedia article Specific Impulse we get a specific impulse of 12,285 s. If we use the cruise SFC of 15.6 mg/N-s, we get a specific impulse of 6,536 s. Is this an effect of the lower density of air, or simply of the lower fuel required in the cruise? I don't know.