2001 astro 4.3 cat converter

[BOBMC]

i have been told by a mechanic that the problem us caused by a unuvesal cat cnverter that has replaced the original dual cat converter. the location fo the sensor is incorrect and the computer reads incorrectly. local chevy dealer confirms (as expected). Problem started as a P0101 intermittant, low fuel mileage, poor prerformance in the 55-70 mph range. traded mass air sensor from my other (99 year) no change, muffler guy measered pressure and said slight over pressure, noticeable at speed only probably. the p0101 went away with a C & EV sensor monitors not reporting. Drove about a 1k miles and service light solid again. This tiem I took to local mech that I usually trust and got he original part story above. Any creedance to this? The Mass air sen off the 2001 is working just fine on the 99 modle.