Has anyone ever replaced their OEM exhaust with a custom exhaust on a 90's Seville\Eldorado? I'm curious what the cost is to upgrade the exhaust and what the performance difference might be...

you found the right place. there's a few of us here with exhaust work done. on my 91, i'm running an 18" resonator in place of stock cat and (previously no cat) and dual out magnaflows. i got the muffler and installation of everything for $260 plus tax. 2.5" pipe all the way.

any camaro style muffler can be fit to your car. you can get the dual look by, obviously, getting a dual outlet muffler. if you just want to use aftermarket muffler and retain stock outlet, get 2.5" in 2.25" single out.

go here http://www.cardomain.com/memberpage/472581/3 and see my exhuast on my deville same setup on your car,this exhuast was originally on my 91 eldo i had first, if you also want to here my exhaust go to page 6 and click on the links. almost forgot, $200 buck total for my exhuast,did it myself took about 30 minutes,muffler that i have on it is a Edelbrock Rpm Series muffler $150,it's a camaro style muffler,very nice deep tone and aggersive when you want it to be.slips right on the stock pipe.so basically a cut and clamp job.