Have lots of smaller questions that the search engine didn't help much with.

1 - I had the cat converter and muffler removed last year and a piece of 3" stuck between. I understand this should probably be changed. Can I change this to a 3" or 4" myself (driveway and jack stands)? Anyone have any tips?

2 - Is the stock turbo control good enough or should I be looking at a TurboMaster or something like?

3 - Can someone explain the benefit of the #9 resistor? Is this something I should be doing?

This truck is now up to 235000kms and is feeling old and tired. Anyone have any "wisdom" to make this truck feel younger again besides a new Duramax?

1 - I had the cat converter and muffler removed last year and a piece of 3" stuck between. I understand this should probably be changed. Can I change this to a 3" or 4" myself (driveway and jack stands)? Anyone have any tips?

You could but an after market exhaust would be much simpler and probably sound better. I Like Magnaflow, sounds sweet.

2 - Is the stock turbo control good enough or should I be looking at a TurboMaster or something like?

If it works leave it alone. If you like tweaking on it and have gauges go for it. Could stress your engine if you go overboard.

3 - Can someone explain the benefit of the #9 resistor? Is this something I should be doing?

#9 will slightly increase the maximum fuel output. Benefit small amount of power increase, may not be noticeable on your high mileage engine. Resister is used in calibrating the injection pump and normally they take a #5. Used for small incremental adjustment.

4 -This truck is now up to 235000kms and is feeling old and tired. Anyone have any "wisdom" to make this truck feel younger again besides a new Duramax?

New set of glowplugs, new set of stock Delphi injectors, fuel filter, airfilter, replace worn tires, and have the frontend serviced with new tie rods, ball joints will improve drive feel.

Nothing wrong with the turbo-master( equalizes boost pressure better than wastegate) and leave the muffler and kitty gone unless you don't like the noise. I have 3" from turbo back to cab and "Y"d to 4" duals and it sounds pretty good to me and anyone else that has been around it. Pretty quiet at idle and on long cruise, but at acceleration and first cold start up it can be a bit loud. Kind of a combo of Flowmaster/Magnaflow/custom job.
Mark