Hey mate,
I regularly tow a 19ft boat (2 tonne) with my Nav and it goes alright. The new suspension made a huge difference as it now sits flatter, more solid, and doesn't bounce around on the highway. I would recommend this modification if you're going some distance.
Take it easy if in 5th gear around 70-100km/hr and change gears as soon as it starts to struggle. Should tow all day long!
Guido