Dave, you are correct in that we're talking about two separate patterns here. This whole conversation got me curious, so I e-mailed Mike Mercer to ask him directly about the pattern and its origin. Here's his response:
Thanks for the interest in my pattern - always flattering! Gary's Deep Sparkle Pupa far outdates my pattern, which I just developed in the early 90's. I've always had fantastic luck with Gary's fly, and wanted to use what I believe to be a benchmark tying technique (Gary's use of the antron veil) in a pattern of my own - consequently, the Sparkle Quill Pupa. It's certainly not a better fly than Gary's, but sometimes works as well or better in streams where his fly is used heavily. I'm a big believer in always showing fish something fresh. Glad the fly has caught a few fish for you - nothing makes me feel better!