I'm afraid you are somewhat chasing your tail here. P0420 (and P0430) are codes for aging catalytic converters. Long before any sensors were changed, your tech needed to run a scan on the voltages of the before and after cat sensors; if the sensors are running at the correct voltages, there is nothing wrong with them. In my experience, gas additives and driving the car hard are band aids that seldom work. Do a search on these codes, this comes up from time to time, usually with the same end result.
