As far as grey market, I've never been asked by Tamiya for proof of purchase.
This might have been true in the past, but one of our tankers from the San Diego club was asked to produce a domestic proof of purchase for an item he sent in to Tamiya USA before they would consider the item for repair or replacement. This policy seems bolstered by the fact that Tamiya's grey market web page is a fairly recent edition to their website.
Times are changing it seems...