my system is 2.5" with extractors and twin resonators, lol no cat, no muffler
but as a member on here who USED to own a navara, the system i would personally get is a 2.5" from the turbo back (new dump pipe) with no cat and no muffler.
i believe running no muffler may get a bit of drone and may be a bit noisy for some, but if it was too loud i'd simply have a muffler put into the system, and it'd be a high flow/straight through muffler at that.
i reckon 3" is just too big for a 2.5L engine, but thats my opinion. a 3" on a 4.2L TD, definitely.