To answer your question, yes a steroid cycle would enhance your results with p90x. My father went through p90x last summer and I did several workouts with him to help motivate him, so i know what it entails. The program requires a lot of endurance and is good for older men looking to lose fat while learning to strength train. Honestly, the exercise movements in the program are quality and diverse. They could be implemented in any workout program successfully.
However, the premise behind p90x is not geared towards building muscle mass. Instead it focuses on high volume (many reps) and lighter weights with little rest. It's optimized for burning calories. You could do some of the same moves in your own program and shoot for 7-8 reps with heavier weight with a little longer break between sets and see better mass building results in my opinion. But yes a cycle would definitely boost your results if your set on p90x compared to not being on cycle. But I'd suggest a different program for mass building.