think you would be hard pushed to notice a difference to be honest, i would say the engine is quieter and pulls a little better, prob due to the a twin turbo setup.
mpg is roughly same or slightly better better with D40, although i haven't done much motorway towing with np300,averages about 25-26mpg
The New Navara is far better at towing. I have had a two D40’s and now have the new Navara. The notable difference is the 7 speed gearbox and the twin turbo giving you a more relaxed towing experience. I tow a 7m twin axle and don’t see any suspension sagging. The mpg is pretty much the same as the D40 around 21 - 23 mpg with an auto box.
I don’t think you will be dissapointed unless you have a V6 D40 then I can’t compare.