Most street engines need backpressure to give a proper street torque curve, and each car/motorcycle's exhaust system is a series of checks and balances, designed with almost everything else first and power last.
Open pipes are loud, smell atrocicous, and often only make additional power in the upper revs. Try to go out for an evening on the town, and while the engine is warming fiddle with the trunk. Forget your Issey, you now smell like a mechanic
Apparently, the M5's primary "light off" forward cats are the primary source of restriction, and eliminating those seem to give the most power benefit. I'm sure a small amount of low end suffers here too, but as the revs build the exhaust system fills and proper backpressure is acheived.
ASR Miami has an exceptional, complete exhaust system, which is purported to achieve good results in power and sounds great in person.
Hi-flow cats are a solution to a problem for those who want extra power w/o running afoul of the law (I believe there are still laws being broken by messing with anything related to emissions, just less with them unless certifed, blah-blah-I'm not sure). On the M5, it might help you get some extra power w/o massive loss of low end. Higher flow cats and straight pipes usually shine in a road race, where heat buildup is a big issue-removal of those heat potatoes at the end of the M5 headers should really reduce heat buildup close to the engine.
I have heard of random smog checks points in other states, though I'm, not sure how realistic that is (if you take a cross country/state trip, etc.)
Sorry for the book-