Just like the title says just bought an HKS "silent power" cat back system for my II gsr. What they like, ? any one got one ?, what kind of gains can i expect, are they really silent ??(hope not too much ). Would love to hear what you all have to say and would also like advice on the right filter to go with it and also weather i could use a de-cat pipe with the standard down pipe and my after market HKS unit.