Here is my take...I assume you are refering to the "halo" area above the pipe.
In the pics below you will see I own an Avalanche to which I installed a thru the body Corsa exhaust pipe.
The point is...the heat generated by that exhaust on Avalanche cladding and the product I use to keep discoloration away and the success I have is related to a product which may be of some interest to you as well...or not.
First I would suggest you try something far less expensive to "test" and see if indeed what you are experiencing can be "corrected".
I believe what is happening is the heat of the exhaust pipe is drying out the plastic. Chevy learned a pretty important lesson with the Avalanche...yet is fixable if you know how to deal with it. What I think is happening to your plastic is different yet similar.
First I would suggest you take Simple Green and a stiff bristle brush and thorughly clean the entire plastic bumper area. ALL of it. Clean it well, scrubing with the brush and rinsing completely and drying with a towel.
303 products are very good but wash off rather quickly and may streak with exposure to rain etc
To save money try a tire dressing on the plastic which has 303 in it. Eagle One is a good one to test with. Dress the entire bumper area with it. Let it set for an hour. Then come back and wipe off any excess with a microfiber towel. If this treatment lasts for 4-6 weeks and keeps the discoloration from returning then the next product may be of interest to you.
We have tried them all...
Refinish Restorer is by far the best IMHO
There is a consensus that this product is a Varnish/Linseed oil based product
It IS expensive
It DOES last a long long time...over a year between applications
It leaves a HARD (almost like clear nail polish) shiny finish to moldings
It requires a very small amount for coverage-one can will last years on the NOX
It smells really bad and should only be used in a well ventilated area
It needs time to dry between application and driving (a couple hours at least)
It is amazing
Here is a link to order...
http://yhst-13811118617756.stores.yahoo.net/carere1l.html
I included a pic of our Avalanche so you can see what this stuff looks like
FYI...Our Av is a 2004 which we have had for 7 years and IS NOT garaged
I think the picture speaks for itself...
The pic expands...how it expands depends on your browser