I think that at 6" you shouldn't have a problem. While you are tall, you are still within the normal range of height I think. Test it out to make sure, but I don't think you have anything to worry about.
I'm just worried about the shape of the tank which locks your knees against it. I'm in the process of getting my motorcycle gear together then will most definitely arrange a test ride and post my impressions here.
I didn't think there were big differences in size between the different Duke models. Are the dimensions really that different? Having your knees against the tank can help you to get more grip on the bike, so i don't see it necessarily as a bad thing.