ive got one off there universal ones. over all they are acceptable. the newer (for newer model bike) the tank the better it will be. it all depends on the condition of their tooling. some of their moulds are very old and have been repaired many times so the seams can be a little rough (they told me this as mine was a little rough) i would say that their quality is good but probably not as good (they are not terrible though) as Acerbis or RV aqualine but they are $600 AU plus tanks where as most of the Clarke ones work out around $250-$320 AU or even better when the exchange rate is good or even cheaper when you buy them from motorcycle accessories direct on ebay who are around $10/$20 cheaper on all the tanks.
their replica tanks should be moulded from originals so they should be right
i suggest you email Clarke and ask them many questions before buying. they will be happy to help you