I'm certainly no expert, and it's been a while since I had to know about this kind of thing, but it's not as simple as just paying the duty. Customs can impound the item until you've proven duty had been paid and technically avoiding duty is a criminal offence so it's not as clear as that.
Also, given that duty hasn't been paid then you're not the 'owner' of the goods, should your insurers discover this then they are perfectly within their rights not to pay any subsequent claim.
I'm not saying that any of the above will happen, the chances are slight, but you should know that there's no such thing as a free lunch and cheapness comes at a price. You just have to work out whether the price is worth paying, hence my hesitation on buying from Kerso.
Also, although I know that Nikon offer international warrenties on their lenses, has anyone had any experience of using one with a Kerso/grey import lens?