There are two common things that can cause this. First, check the torque/tightness on your pipes at the head (acorn nuts). Make sure they are properly torqued, sometimes that can cause an air leak and popping on decel. Also, did you replace the crush gaskets when the pipes were changed out? If not, that could also be a potential problem.
If that all checks out, I would check to see if the PAIR valve has been disabled or removed. I don't have an 1800 and could be wrong, but I believe that they come with one from the factory. If it isn't disabled or removed, get rid of it. It's just smog/emissions junk they put on at the factory to meet EPA/emissions standards. If you remove it, you'll need to order some reed valve block off plates (usually around $13 from Baron's Customs or some of the other aftermarket suppliers). This is another common cause with decel popping in the exhaust when you are running aftermarket pipes and an aftermarket air kit/intake.
Hope this helps! BTW, they have to find something to knit pick since you spent 1/3 of the money on a bike that's faster than theirs!