are aftermarket Cats okay on a 01 E320

[davidlee]

recently I heard a rattling on my girlfriends car, so we took it into the exhaust shop and they said that the rear catalytic converters both are bad. we recently had the spark plugs replaced because we were getting a slight misfire at idle and the mechanic told us that the plugs were due to be changed anyways but the Miss fire was caused by a loose coil pack. The Shop told me that due to the misfire it could have prematurely damaged the cats.
replacement cats through mercedes with piping are 600 a side. Plus install
I'm a person who saves money for rainy days but at the same time does not spend money it does not have to be as I am on a budget like most nowadays.
my question to you is are Magnaflow high-performance catalytic converters okay to run in replacement of the Mercedes ones or am I going to have big problems with early failure and need to be replacing them soon again.