There's nothing like resurrecting a 4 year old thread! [thumbsup]
Just in case somebody does a search on 'Wileyco' I thought I'd set the record straight.
I don't know what the guy who said Wileycos are truck mufflers is talking about, but Wileyco is an Italian manufacturer of exhaust systems. Rossi had them on his bike when he rode for Honda. He won the world championship, so I guess they might be "performance oriented". At one point, Wileyco even made a limited-edition Rossi pipe. Even in Europe they seem to be a smaller company, but in the US, they are downright obscure. By the way, it is pronounced vee-lay-co.
I had a pair of stainless Wileyco slip-ons on my (now sold) Monster and they sounded great and held up great. They have a deeper sound than many pipes and not a hint of the tinniness that you often hear. I had a MTS with the DP/Termi kit while I still had my Monster and I liked the Wileycos better. My current Multistrada has a full Remus system and I haven't quite figured out which I like better. The Wileycos were a little deeper down low, just off idle, but the Remus system sounds downright nasty WFO.
I'm not sure how they'll sound on your Monster but a co-worker of mine has some on a Suzuki SV1000 and they look/sound great. I also can't comment on the performance of these cans because It's not my bike. Overall he's very pleased with his purchase so I guess that counts for something.