Hey 8bender - The HP ratings have never been a strong point for the early U.S. 928 models. Although I have always kept Miss Purdy (84 928S) tuned and in tip top condition - plus an especially thorough once over just prior to any out of town trips, a serious boost in HP would be a welcome improvement.
Like most 928 owners, we're often the leader of the pack out on interstate and perform fine over most mountain passes. But, when I was into some of the steeper back road twisties of the North Carolina - Smokey Mountains, she was breathing hard just to stay up with traffic. Of course it didn't help negotiating a blind switch back while trying to stay out of the way of the locals who were obviously familiar with every bump in the road. At times their advantage really kept Miss Purdy huffing and puffing when exiting a tight curve and passing my own back bumper. It was especially dangerous at night since NC doesn't seem to waste much money on guard rails or paint for signs posting a suggested safe speed along the edge of many of their dark two lane roads.
If there are any secrete formulas (short of an late model LS transplant) that boosts our HP please post the information - Miss Purdy is definitely in need of some help
Michael