I emailed Scotts rods re their different ones, they know their stuff, different dump pipes for the different models, got a fast response to my questions, seems like a good thing.
I have bugger all experience with Navara's, however all Turbo engines are the same when it comes to exhausts.
After the turbo, other than stopping rocks from falling into your snail, the exhaust pipe is only there to get the fumes and heat out from under the car, BIGGER is better, no exhaust at all would be the ultimate for power, a turbo diesel has no requirement for any form of back pressure, only NA engines need this, obvously there is a point when going bigger makes no difference or improvement in power, however, with late model stuff fitted with DPF's and Cats, the size of your system wont make much difference if there is a blockage in the pipe, which is what a cat is, it restricts the flow of gases to increase the temp of the cat so all the nasties are burned inside, good for the enviroment, BAD for power, bad for fuel economy.
If you wan a cat in your system to stay legal, buy a good 4" metal cat, they are much less restrictive (you can see through them) and flow heaps, put this in a good mandrel bent 2.5" (min size) or 3" (best) with good high flow (straight through) mufflers, and this will allow you to stay legal and not slow you down at all.
I have a 3" mandrel bent, 4" metal cat, one high flow straight through muffler on my cruiser, it is great, HUGE increase in power, better fuel econ and almost instant spool up of the turbo.
I have also used the same set up on turbo falcons, I know, not a TD, but a 4 lt engine, running to 6500 rpm, punching out 350kw at the back tyres, over 420kw at the flywheel with over 1200nm of torque with a 3" System and 4" metal cat, if it can flow enough for that, a 2.5lt TD wont even go close to stressing it.
I have not looked under the bonnet yet, but I dont think the turbo has a wastegate, so a HUGE dump pipe wont make a lot of difference but remember that the gasses coming out are very hot, hot gasses expand and need lots of space to go away fast, so even if you only want a 2.5" system, I would use a 3" dump, onto the cat, then drop down from there if you like, as the gasses cool, less room is required.
Good Cats and mufflers are what you should be looking for. buy the best you can get.