Yeah, I agree - a 38mm J (or any J, for that matter) is a little much for that rocket. I don't think you'd ever see it again. If it's getting 1500 feet on a G64, I don't even want to think about the altitude you'd get on a motor 6 times as powerful. Not to mention the stability issue. The 3" Horizon would be fine for L1 on an H128, H180, H238, or even an H165. For L2, definitely go with a bigger rocket.
Loopy