There are two 3.5's: The regular one and the HO. The major differences are the HO has slightly lower compression, larger turbos, and more aggressive tuning.
As for octane, the truck has a very accurate knock sensor and monitors knock closely, and adjust the timing and fuel curves accordingly. There is something like a 20-25 hp difference between regular and premium fuel on an otherwise stock EcoBoost. Much to the dismay of raptor and Limited owners, for about 600.00 you can easily tune a regular 3.5 to make more power than the 3.5HO.
I ran a set of CR stage 3 cores in my stock turbo housings on my 2015 and had meth injection. It was probably close to 475-500Hp at the rear wheels. It held up well for the 240k that I had it. It could spin all 4-35x10.5s on dry pavement without much effort.
The 3.5 EcoBoost is based on the old NA 3.5. The 2.7 was designed from the ground up to be a twin turbo, full size truck engine, so it’s internals are very stout. I almost went with the 2.7, but they were brand new when I bought my F150 and the 3.5 was a little more proven.