Of course, you'll need more flash power because the ND filter will knock down your flash power along with your ambient. However, with studio strobes, you've likely got the extra power to make it happen. I took the following shot this weekend using a CPL at just about full polarization (which makes you lose about 1.5 stops of light). I was using a relatively low power White Lightning Ultra 600 with a highly efficient Paul C. Buff 22" High Output Beauty Dish.


At f/6.3, ISO 100, 1/250 sec, and the CPL, I was able to bring down the 9:30am ambient sunlight significantly.