That seems to be one of the worries about the new diesels coming out designed to run on the ulsd. Apparently emissions are so strict that there are all sorts of filters and scrubbers in the exhaust to try to keep the crap from getting to the environment....
no cat.. i got a code once for insufficient egr flow. when i plugged into it the checkengine light went away. im sure it will come back again. i was pulling a fairly decent hill when the light came on. once in about 5000 miles now.
There is a little device you can buy to keep the code from setting called a finger stick, there are only two places you can get them, from the guy who makes them (Fingers) and from kennedydiesel.com im prety sure they are like $20 na dthey are not hard to install and after you instll that you can put a blocker plate in the egr also! if you blocked the egr with out a finger stick the light would come on all the time because it would not be getting anyflow at all!
I believe that the muffler is more restrictive than the cat, but you can't really hurt by taking it out. The gains won't be significant, or noticeable powerwise but it should help give you even more EGT reduction.
EGT's might get "high" depending on what programmer you go with. But "high" EGT is all to often viewed as excessive by some, when in reality high EGT's in short bursts will not do any damage to anything on your stock truck. The exhaust mods will help keep it cool.
If you get out of a comfort zone on your egt provided you have a gauge to view it you can either use a lower setting or you can throw on a small shot of nitrous. Or you could get a snow performance water/meth boost cooler...
Propane from what I have heard and read isn't really the best choice. You can get better mpg's with mods such as intake, exhaust, and a chip that isn't set on it's highest level but just "economy" mode if it has such an option. That along with light footed driving practices make for great mileage.....and leave your tailgate up too! You'll have to think whether the investment in the entire propane system with the tank (which can be expensive) and the cost of the propane fillups if you will actually gain much if anything in the long run. If you only get 1mpg more with it how long will it take to "pay" for the benefit? As far as power it supposedly works well, but again it's not the top choice with many.
N2O has been proven very dangerous to the dmax, almost all of the dmax engines i'v seen with holes in them was from to many drugs, Water/meth and propane are alright in extreem moderation but have still caused blown engines! The duramax is a very different animal, you can get great miliage and power and driveability all with just tuning and other minor mods. unless your looking for that last hp and can risk popping an engine i would stay away from the drugs!