I understand what you think the logic is, but it ain't logical. First go and get a baseline of how it performs exactly like it is now (not like it was a few months ago), then spend the rest of an hour of dyno lease (with A/F monitoring) finding the optimum timing and mixture.

If you "do almost everything first," you'll have no idea whether those changes are helping, hurting, or just expensive. Knowing stuff like that for a fact is the whole point of dyno tuning. I can't see any reason why octane booster would be necessary or beneficial -- it would just be another variable with an effect you wouldn't be able to judge. Same with the spacers, etc.

It does make sense to fix anything you know is wrong before you go, of course -- unsynched carbs, or whatever. Get what you already have optimized, then go back another time with ONE thing different -- spacers, perhaps -- and see if that did anything. Then add the horns and check that. Then take the spacers out and leave the horns -- that may or may not be better.

You get the idea...