A quick note for anyone who plays this game and gets to this point without resorting to Magic Engine chicanery...
I've found that if I have the turbo switch for button II in the "middle" position, my guns will maintain their streams of fire. I thought I had tried this earlier unsuccessfully, but I guess I hadn't. The other problems I noted--significant slowdown, disconcerting changes in speed, and occasional self-pausing--are still present, but at least I don't feel like a sitting duck now, and I'm definitely doing a lot better with it.