I hope you're going to a BBC to match those 2" headers!
FM muffs have gotten a bad reputation. Whether it's deserved or not, I cannot personally say, having never run them back-to-back w/other muffs w/no other changes. If you believe the manufacturers of other
mufflers, the FM's suck. There might be unbiased tests that have been done, I'd only use that, not ANY manufacturer's published numbers. Numbers can be made to look however you want them to look. Old saying: There are lies, damned lies
, and there are statistics
. Same thing holds true here IMO.