Sooty tailpipes after dual CAT replacement on STR

Hi. I have recently replaced both CATs on my STR. And now both exhaust tailpipes are accumulating sooty deposits . Originally the left bank was failing, causing the EML to be displayed,  but only the left tailpipe was sooty. The right side was clean. Since the replacements were fitted, both of which are genuine parts, both tailpipes are black. The car doesn't emit excessive smoke either at idle or under acceleration. It gets an 8 mile run to work daily, and traffic permitting, is opened up when safe and appropriate to do so!. Fuel consumption also seems to have dropped by 10 - 15 percent. My driving patterns are the same as before and after. I run the car on premium unleaded. It has done 76000 miles. Any suggestions please.