Single exhaust will not impede performance, I have not noticed any loss of power with single exhaust. The scavenging effect is what helps the engine breathe. Granted the exhaust needs to be properly sized whether it's dual or single. If it's dual an h-pipe or x-pipe adds the scavenging effect back into the system. As
@4WDKC mentioned, an exhaust shop may be the best route to take for a y-pipe, but be sure to ask how they make them/route them. I took in my truck to have the exhaust done, and the y-pipe they made was a pile of crap. Sure, they thought it was great because it wrapped around the front of the oil pan between the cross member and then y'd into the collector they made alongside the transmission, and cut my frame because the new pipe was close, yet there was a stamped-in high point for the pipe go under, they just didn't route it under that point. It got in the way of preventative maintenance things, like changing the oil filter and oil, the starter, etc. It looked cool, but I ended up taking it to another shop to have them make a better y-pipe. The original shop just wanted to argue with me that their y-pipe configuration wouldn't cause any harm to the brake fluid because the pipe was within a 1/2 inch from the brake lines. I told them that was a bunch of crap, my truck is not a trailer queen and I don't want the brakes failing because of boiling brake fluid while hauling a load. It may not have happened, but I wasn't going to take any chances. They also didn't see an issue of changing the oil and letting oil drain on the pipe because it would burn off. A stock y-pipe routing is definitely best. The attached photo is not from my truck, they routed it to the other side on mine, but it shows exactly what you don't want in a y-pipe.
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