Every engine is a little different. If you can get by with 89 octane at 14 degrees advance and get NO detonation, it might be fine until the summer months when it gets hot outside. Once the ambient air temperature goes up, so does your intake air charge temperature. The difference between 40 degrees outside and 90 degrees can be enough to require 91+ octane at 14 degrees advance.
That timing adjustor is adjustable for a reason! You might be able to get a few more RWHP out of your engine my bumping advance to 15 or 15.5 degrees and running super-premium fuel (93+ octane). You did buy your adjustor for maximizing performance, right? That, and the web site and instructions were pretty clear in telling you that premium fuel (and the $1.90/gallon pricetag) were a necessity.