This MSD (8455) uses the stock ignition correct? In that case, the timing curve should be a direct match for what you were previously using (there's nothing fancy in there, the hall-effect sensor (PIP) sends the position and RPM information to the computer, and your initial timing is about all there is to adjust). Sounds like you've had >10 (pinging), and now 8 (sluggish), and you preferred it at 10. Set it back to 10 would be my suggestion too.
Most places around here, 89 is only 10cents a gallon more than 87 (3%). You might even see that much of a gain in mileage if you bump the timing back to 10-12. Just a thought.
Other than that, you can tune a different timing curve (stock advance or a little more at idle / low RPM-load, and then dial it back at higher loads to cut the pinging). But that's an expensive solution to a simple problem.