Your car has knock sensors that will have your PCM adjust the timing accordingly and prevent damaging pre-detonation. So, as long as you are not flooring it all the time, 87 should be fine. The manual only recommends 91 for best performance. Not required.
I've driven a number of turbo charged cars, and in my experience, if you drive normally (no aggressive driving/keep rpms down) running regular gas is fine, as, as stated above, the ecu can make up for it in how it advances/retards the timing. However, you will get less horse power than what is stated. And aggressive driving can cause pinging, but unless you're trying to race on it, you probably won't cause damage.
as far as running higher octane gas than is recommended will net you 0 benefit.
91 is just fine on a stock engine. There may be some aftermarket tunes when combined with certain environmental conditions ( meaning really hot weather ) when 93 will offer some advantage, but on a stock car it's not going too.
You could use 87 if you are going to be very light on the throttle. Don't think that the knock sensor will save you at full throttle, it may not, and yes, we tested this a few years ago with the 500 Abarth.
US Federal laws require that all cars manufactured for sale in the US MUST be able to run on regular unleaded. I don't think there is a single turbo car in the US however that does not recommend premium fuel in their cars.
I realize that the added cost of premium over regular should be considered a worthwhile "cross to bear" in the operation of performance engines. Yet, I'm by nature sensitive to the cost differential, mainly to the change in the premium pricing of 93 octane vs 87 octane. Anecdotally, in the SE US the difference 15 yrs ago was perhaps 5% ($2.10 premium vs $2.00 regular). The differential has risen steadily since 2008 or so to 25% or more ($2.50 premium vs $2.00 regular). I am curious as to the performance and longevity compromises affecting turbo applications using 87 octane instead of 91+, as I have used in the past 20 yrs running BMWs, VWs and Saab and Volvo turbos.
As an example, the VW TSI turbo application specifies 87 octane. Is this system then essentially a neutered turbo application, in which the ECU map's top priority is to resist/minimize/prevent detonation from occurring, and realizing whatever (increased) performance the turbo provides under those constraints?