Running 89/93 on an engine that does not require it is throwing money in the trash. The reason why some cars need to run the higher octane is because of their compression ratio. The lower the octane, the easier the fuel is to combust. With an engine running a high compression ration, that equals knock which is bad news. You want a fuel that will not combust as easily since it is being compressed further than usual. There is no power gain behind it, just the fact that certain engines need it to avoid predetonation. If your car does not require 89 or 93, it will not make a difference performance-wise. Think of higher octane as greater knock resistance. All 89/93 fuel would do on an engine that requires 87 would do is give you slight better fuel mileage since the fuel does not burn as quickly. However, that is pointless because you would probably save more running standard octane compared to the pricier mid-grade or premium.
However, the ECU tune will definitly give you a noticable gain in power, albeit a smidgeon less fuel economy. Why the higher octane, you ask? When you start messing with the spark/ignition varibles and fuel/air ratio (I'm guessing this is what the tune consists of) among other factors, you're going to need a fuel that will not detonate as easily. That's why with aggressive tuning, you need the higher octane. Even though the compression ratio isn't changed (only different pistons will do that for ya), you'll need premium fuel since the spark is advanced. Still, like I said before, unless you need the higher octane, running it in a regular engine is like throwing money in the trash. The ECU tune however will definitly help with get up-n go and I think it's worth paying more at the pump to have a happier Focus :D
Lastly, in regards to octane booster, same thing applies. Unless you really need it, don't use it. The only time I have ever purchased it was when my dad drove his Mustang down to Mexico for vacation. 89 octane was not available so we had to settle with 87 and a bottle of STP. Worked well, no knock. However, I wouldn't use it as a substitute for buying higher octane at the pump, as you'll be paying more for the booster than the fuel anyways. That stuff is mighty expensive
Just a poor college student dreaming of owning a ZX3.
"I'm not going to take this defeatist attitude and listen to all this crap any more from all these people who have nothing except doomsday to predict. "
- Carroll Shelby