If you can burn a tank of fuel in a month, you're probably fine. Just make sure to burn the whole tank off, not a half tank and then refill. And since you live in HI, gas line freeze won't be a problem.
As far as gas additives to boost performance ... it's generally all marketing hoopla. In terms of octane booster, yeah, they'll boost octane, but you'll need so many cans of the stuff of many brands of octane boost that you might as well just fill your tank exclusively with octane boost to get a measurable effect (which, I'm sure, probably has negative consequences of its own).
The only real advantage to some gas additives is in terms of detergent effects, to help keep injectors clean and whatnot. I know there is debate in that department as well, though, in terms of how effective these products are and which ones are best. And many brands of gasoline already contain these additives at the pump (some brands more than others), making autoparts store bottles of additive somewhat moot.