Welcome to the forum!
Is yours a manual? They have a first gear that's probably a little too tall and many people put the suggested 265/75R16s on which have a 5% larger rolling diameter, increasing the effective gear ratio and making it harder for the engine again.
There are several products that people have tried and suggest, from high-end chips like Unichip to throttle input modifiers like Windboosters. Other options are diesel gas or extremes like alcohol/petrol/water injection.
The basic cause of the lag is the design of the turbocharger and more specifically its control. Turbochargers are just air pumps and Nissan chose in the D40 to get even more air flow (higher pressure) in a smaller rev range - providing more horsepower across a more narrow rev range. Take a D22 for a drive - it's actually more pleasant in terms of power delivery, but doesn't produce as much boost as a D40 and as a result the similar engine (they both have YD25 plants with minor differences in head and injectors) doesn't produce as much power - it just comes on more smoothly.
The dramatic increase in power you experience in the D40 is only possible when the turbocharger has reached enough rpm (takes about 1600rpm to produce enough exhaust gas to get the turbo spinning fast enough to start, 1800rpm to make it strong). If you take off with about 2000rpm - 2500rpm you'll find power delivery more acceptable with less lag - but in a manual that will cook your clutch fairly quickly.
It's possible to take the D40 to a workshop that can cook up a new boost mapping for the turbocharger. The D40 has a variable vane turbocharger (which extends the performance range by adjusting the exhaust stream across the turbine) and it's possible to have the mapping adjusted to bring it up a bit sooner, but not by a lot - you need exhaust flow to drive the turbocharger, and you get that from revs & fuel input.
So the best solution does depend on how you want to achieve it and what you want to invest to do it. Personally I tow 2.5T and have hauled that up some of our steepest inclines - try hauling your van up the ranges north of Gloucester on Thunderbolts Way, there are some 14% grades there that really make the car work hard. In my case, I have an auto and just let the car do the work as it can, rather than possibly overstressing it. And mine's approaching 250,000km on the clock - and still feels smooth. I'm having a fuel issue at the moment - not sure if it's SCV or fuel rail pressure sensor doing it - but the engine and gearbox are flawless.