s5 Vert exhaust question for the gurus.
 

I purchased a '89 vert back in december '06 and the downpipe has glowed everytime I run the car. I doesn't do it at idle, but under load. I mean it glows very red. Talking to the previous owner, he stated it didn't do this until he made the exhaust a free flowing exhaust by adding free flowing cats(empty on the insides).

My question is, could this straight thru exhaust be causing my car to run lean and make the downpipe glow? would it hurt to bump up the injector size to dump more fuel into the motor?