Perhaps the newer 4 cat system is more prone to CEL's when upgraded compared to the older system? I hear something about oxygen sensors on the cats or is it headers but I'm not sure if bypass pipes or new cats would trigger a CEL or not. Is a CEL really even all that bad if you know it's due to the new bolt-ons? Does it affect performance when the light is on?

I think it is more critical although
 

the pre-cats don't do much I hear. The dual 02 placement might be critical though. You want all your safety measures still in place even with mods. This picture convinced me that these cats are the way to go (200cell and dual 02).

What brand are they?

They are Roock Motorsports..
 

I also found these supersprints (look similar except the higher polish). I am really fed up with the pricing of euro stuff. If I can gather enough info I will have some made. I need info on the stock size tubing etc. If I can source the cats (working on it), the tubing is no big deal, neither are the fittings. I am sure extentions could be made for the 02 sensors to be relocated. Maybe I am mad (he's mad I tell you)!!, but I really choke on 1300-1800 for a set of cats from often unreliable sources. Anyway that's my gripe for the day:D I feel better now.

So these two seem to be the best option because
1. They are 200 cell instaed of 100 (these are not turbo cars) we need some backpressure right.
2.They allow for relocation of two 02 sensors (critical for later cars).