Catalytic converter replacement - are Dorman Cat Converters good/ worth installation?

Hello Scotty:  

Thanks for producing your great show!  I have a 2010 Mazda 6 with a manual trans, a 2.5 litre four cylinder and 325,000 km (202,000 miles). My check engine light came on (was not flashing) a month ago and was diagnosed as a P0139 Bank 1 Sensor 2 sensor circuit slow response. The second O2 sensor was replaced with a new NTK sensor and the check engine light was cleared. The existing front O2 sensor was not changed. Two weeks later the check engine light came on again, this time with a P0420 code (P0421 for Mazda). A road test monitoring the 02 sensors showed that under cruising condition the converter works but on acceleration the converter is weak.  The recommendation was to replace the catalytic converter manifold unit and the old front O2 senor.  My mechanic quoted me the cost for a new NAPA unit at $3,201 ($2,352 USD) with gasket and bolts, plus the new front O2 sensor and installation cost.    

Question - Are aftermarket catalytic converters worth buying and will they work? 

Looking on line I found that Dorman has a complete manifold cat converter unit(#674-075) for my car complete with gasket & bolts for $891.99 USD ($1,179 CDN) plus shipping and taxes. This is significantly less than the NAPA unit. The NAPA and other units such as Magnaflow are good but are also very expensive and designed to run for many years.  I have been told that cheaper aftermarket converters are not worth purchasing as they only last a short time and usually throw a check engine light.   

My Issue:

I am retired and just want to drive the car for another 2-3 years, then buy a used Mazda after car prices drop, while saving money over that time to put towards a car purchase. My car is in good condition having used Mobil 1 synthetic oil in the engine and gearbox, the clutch is in great shape and I Rust Check spray the car each spring so the body looks great.  I just need to get another 2-3 years out of it!  

So could you please tell me what your experience is with after market cat converters and the Dorman product specifically?   I have watched your videos for years and followed your advice many times, so would appreciate it if you could you please provide me with your recommendation on this.   Thanks for your great show and help! 

Cheers, Owen.