In the days before ethanol, 87 octane actually had more energy than 91/93 octane, however the higher octane gasoline allowed higher compression and, with that, higher power.
But now 87 octane MUST contain 10% ethanol by law (total sales of all grades across the country must average 5% although actual numbers are closer to 6-7%)
Oil companies LOVE ethanol for many reasons, not the least of which is that it is cheaper than gasoline.
According to their own figures, gas companies admit there is a 2% loss in power by using 10% ethanol but they say "most people won't notice". Look around their websites and you will find the data.
If you use 87 instead of ethanol free 91 you are losing around 2% and saving, what, about 20 cents a litre? Race fuel costs many times what 91 costs and still nets only 2-4%, and then only with tuning.
As far it being too small an increase to bother with, I don't know about SVs but on a 600 a $1800 exhaust only nets you 3-5% increase in power.
In fact, young Mr. Carlson's super fast 636 only makes about 10% more power than the average 600.
2% ain't much but it's all the little things put together that make a difference.