The Burke's upper-hand stretch is wider, or at least feels wider, than any Low D I've played. I've always used the "piper's grip" for the lower hand due to my background as a Highland piper but I've always used the end pads of the upper-hand fingers. This approach has worked on every pipe or whistle I've ever played over the last 35 years until I got the Burke Pro Viper.
Right away, from the first day, I could feel discomfort in my upper hand. It's a combination, I think, of the Burke's wide spacing and the Burke's huge fat tube. I knew that the solution was the piper's grip for the upper hand but it took a long time to break my upper hand's habits and get it used to the new grip.
I just played a while last night and had very few squeaks. I'm getting to the point where my upper hand can be at the middle-joint pads, or end-joint pads, or anywhere in between and still seal well. That's where I want to be, where I can hold the whistle wherever it's most comfortable and not have to worry about getting a good seal.
By the way the Burke has a great big sound, an exceptionally powerful bottom D, and great tuning over the range.
Carried away by the madness of fight, the English knight charged straight into the Spanish array. Here and there tossed the white plume of the English helmet, rising and falling like the foam upon a wave, until at last it had sunk from view, and another brave man had turned from war to peace.