If you want it to line up properly, you will need the PP belly pan. The lip will fit perfectly but the stock belly pan will not fit properly against the PP lip.
I was thinking about this as well. The PP lip will fit the bumper just fine but will not match up with your belly pan. Depending on how the Steeda lip bolts up this may or may not be a problem. If the Steeda lip bolts up to the bumper or other solid area under the car then it would simply cover up any coverage gaps that your belly pan will leave behind. Now if it uses that belly pan for its structural support then it won't work out as well. Anything is possible with a bit of ingenuity though, perhaps one can run the PP front lip without the belly pan completely. I'm planning on tackling this project later in the year once there are a few more aftermarket options.