I installed a Welter Perfromance stealth exhaust on a 2016.5 Sling a couple of weeks ago. I will note I already had a DDM Cold air intake, no other engine mods. At the same time, I bolted on a 1320 header (Version 1). Wanted to provide a few comments for those considering. Will start with the 1320 install first
1320 V1 HEADER: Seems pretty well made, not gorgeous to look at, bur FAR better than the stock manifold. Install was a breeze, I had no issue with the header not clearing the manifold studs, and in fact it was no where close to interference. I was a little concerned bolting it on, since you can't use a torque wrench, but it worked to well. One negative to the 1320, the bolt hole for the front top is too close to the pipe, and the bolt will not go all the way down. I cut a washer (to C shape), and slid it on so the post would make contact. 1320 should tack on a washer of some sort from the factory. Anyone with any mechanical skills should be able to do.
The Stealth Exhaust: I am VERY please with the system. Install was extremely easy, and went in just like their video shows. This was as easy if not easier than the header! Very well made. Sound may be a little louder than stock when sitting still, certainly deeper between the header and the exhaust. That said, when driving it is quieter unless you really get on it. As for exhaust fumes, I've noticed them once or twice when sitting at a light, and the wind was blowing the right way. They were not strong at all, and when moving I get nothing. I will note I have Corbin bags, so not sure if that will make it better or worse (my guess is worse, since it would collect air under the back end. Would I buy again, absolutely. Only change might be to go for the 2.5 inch in case I go turbo down the road, but not planning on it. I will also note, there is a significant smell for the first 100 miles or so, that then backs off. I would imagine that it is the oils burning off from production -- if you buy one, expect some smell up front that will get better. My 1320 actually smoked for the first 10 minutes or so......
For those wondering, I did my install with the Sling sitting on two ramps under front tires. It was a little tight moving under there, but it does not take too much space to wiggle the pipes above the frame cross members. If you had a higher lift, install would be a pice of cake.
How did the 1320 and Stealth affect performance: I do not have a dyno, but the way it revs is significantly faster. Combined with cold air I already had, the engine runs like it should have all along. I am debating the retune of the computer, but am already happy with the system. It also feels smoother, although that may be an artifact of the where the stock exhaust was, and its sound quality.
What could Welter improve on (and these are very minor!): I wish they had a ceramic coated version in place of wrapped. Living in FL, I am concerned the pipes will rot over time as the wrap holds moisture. No evidence that is the case, though. I wish they would provide written instructions. Their video is GREAT, but I had to go back and forth to the computer several times, when it would have been easier to glance at paper. So far, that is about it. I could have resolved the ceramic coating concern, but did not really think about it.
Would I buy these again, absolutely. I was not after a loud sound, knowing it was a 4 cylinder anyway. I think it is actually more pleasurable to drive, and can hear music much better when moving. Quality product, do wish it was cheaper, but understand that quantities are somewhat limited. Thanks Welter for a good product. 
