The metal changes? How? Changes shape? If it's so weak that the baffles, themselves, change shape from heat and pressure, that's some seriously wimpy steel they're using in their products. If Flowmasters had any sort of packing/insulating material in there, then I could see there being a break-in period, a-la Dynomax UltraFlows and Hooker Maximum Flows (they sounded TOO quiet on mine for the first month or so before the sound packing settled in, then they got wicked under WOT). But unless
Flowmaster's changed their designs yet again, they're just a bunch of holes and triangle deltas of solid metal welded into them - nothing at all to "break in."
I'd say whatever it sounds like now is what it's gonna sound like for good ... until they rust out.
(On a totally unrelated note, the animated silhouette of this iPod Nano chick next to the Quick Reply box is making me crack up. Looks like she's fanning away a fart...
)