I just talked to a guy employed by the Highland Park distillery last week, and he said that the 12YO (latest bottling) is 100% ex-sherry casks. He actually mentioned a percentage of european and american oak, but I don't recall exact numbers. From memory I think it was 80% American and 20% European for the latest 12YO, but I'm not sure. I tried it and must say I was rather impressed, my experience much mirrors that of the OP. The guy from the distillery didn't say whether or not they still have bourbon casks maturing but I'm sure they do.
As a side note I have consumed a bottle of the 1977 21YO Bicentenary Release. I found a bit less Sherry than in the latest 12YO but a lot more peat. BTW; wasn't it 40% abv. and not 43% ?
Also agree that the 16YO 1 litre duty free bottling is sub-par re: the recent 12YO. Save the money and buy the 12YO IMHO.