My exhaust was blowing a fair bit as the V clamp that joins the Piper 4-2-1 to the flexi section had rusted through

After about an hour browsing eBay, a clamp for a Peugeot was purchased, and I fitted it tonight. With the usual combination of some firegum, I now have a pretty decent seal but the car is still really lacking in power, and sounds too loud when under load.
So, when I got back home I thought I'd see if it was leaking again, but it was ok. I then noticed tiny flecks of something coming out of the exahust, then this come out...
It looks sort of honey comb like, but is aboutt he side of a finger nail. Is this my cat disintegrating...? Would this cause a lack of power underload (blocking exhaust flow perhaps?)

