FWIW, I got mine done not too long ago at the station up in Erie/Frederick area right off of Hwy52. I first went in with my aftermarket DP installed just to see how close I could get, and if I could get away with not changing it out with a little E85. The readings were somewhat close, but I failed on one, the NOX if I remember right. Went home, swapped my stock downpipe on, went back and passed with flying colors. I would just hold on to your stock DP and swap it out when you need to pass. Did not receive a visual inspection on either of the tests however.
Shortly before this, I had to take my old 2.5RS with an 02 WRX Swap and a bunch of other supporting mods including an external wastegate which made it a little tougher. I couldnt just borrow a stock DP, so after going to an exhaust shop, having them weld in a new OEM style cat and topping the car off with 3-4 gallons of E85 with a little splash of denatured alcohol to mask, she passed with flying colors as well.