According to the Just gotta Scoot review-
www.justgottascoot.com/symba.htm - ...the optional longer seat helps you use the heel shift. A passage from that review:
"For me, it's like a whole new bike! I was able to sit comfortably in the position that allowed my foot to be perfectly placed on the heel/toe shifter."
I love the stock seat with a milk crate bungeed on to the back rack but it's true, I find it difficult to use the heel shift with my knee forward of my ankle.