Couldn't find a post that answers my question so here it goes. On a '05 SLE car if I was to go with a high flow cat would I gain anything OR do I need to mess with the resonator or down pipe do get full benefits? Do the Bonnies even have a restricted downpipe? I keep reading that the Bonnevilles have a nicely free flowing exhaust system from the factory and was just wondering if there is room for improvement that makes sense.

You don't have the W-body restrictive U-bend.

A downpipe might hurt you with not many mods.
A high-flow cat will help any stock or modded Bonneville.
Keep the resonator and stock mufflers (for now).

Thanks for the info. I will go with only the high flow cat then.

A resonator delete will be on the sound change later. And easy and cheap to do if you prefer.

One more Q. Trying to look up The Carsound/Maganflow cat for the Bonnie and am finding tons of different part numbers. On the Carsound website it lists part # 99306HM for universal and round in shape, other info states part #94136 and oval and several others. Which one do I want?

Have you measured yours yet?

Is there any credible info out there that supports benefits of a high flow cat? Being an engineer I struggle with claims like: I got 5 mpg more after installing a high flow cat, or I can really feel the difference, etc. To feel a performance improvements you'd have to gain a minimum of 5% hp/torque that would mean 10-15hp depending on SSEI or SE, SLE. I am all for perfromance gain as long as there are any. Gaining hp and loosing torque make most of the time "no sense". I can see feeling more power IF you replace a clogged, old cat....

Some of it depends on how bad the OEM cat was to begin with. On my 93, the core rattled loose, so I can't really say there. But on my 95 (also an L67) my Cat was fine, but I put the 2-year old high-flow from my SSEi on that car (when the SSEi got a new one for exhaust cutout reasons). I absolutely felt the difference.

The margin of 'seat of the pants dyno' on our 3800'* (supercharged or not) is about 5-10hp. The Cat is close enough to the manifolds that it actually makes a difference in performance, where mufflers generally don't until you're really modded quite well.

Virtually every car here in the last 5 years has felt a difference going hi-flow. The vast majority that have tried to go cat-less (illegally or legally for the canadians) have felt a LOSS of low-end due to reduced exhaust velocities.

If you do it, I gaurantee you'll feel it.

now we always note that deleting the cat will hurt low end torque. but do we gain high end?

Even if you do, it'* not worth it.

Giving up bottom end on a 3800 is like opening a beer only to lick the cap.