When you did the top engine clean through the intake, did you stall the engine while getting the last bit of fluid out and then let it set for 1/2 hour or more? That is the most effective way of using the top end cleaner. When I did that on my 01 Yukon at 90,000 mile, I had to replace the spark plugs before I noticed better results.
Yes, I stalled the engine and let it sit for a while.
Actually it was a little difficult to turn the engine off: it acted like a diesel and did continue to run even without sparks
After replacing the spark plugs it seems that the mileage is going better, but I honestly don't feel any particular change.
I still have to check the O2's and the cats, however: cats are teh last thing I want to mess with. F-bodies have the cats welded to a Y-pipe and to add pipes as suggested by JR will be pretty difficult...
