I have done the full p90x cycle on Dr. B’s diet–the one minor deviation I do is a 4th meal (protein shake with coconut milk, macadamia nut butter, and 44g protein–still very low carb, but not 100% bernstein). I think the Dr. B diet induces a state where you have lower glycogen stores in general which is going to affect anaerobic performance in general (the cyclical ketogenic diet folks exploit this very fact). Anyhow, I have found that as long as I consume enough calories to cover the intense exertion I have been ok and feel great.
I am now doing intense weight lifting 2-4 times per week (as possible due to time) with cardio 1-2 a week. I like to sprint (all out) every week or so as well. I will probably go back to do some p90x but it is very time consuming and I felt it was a good time to shake things up.