I've run this experiment using my Sienna. I've run a number of
tankfuls of regular or premium fuel in succession so the engine
computer should have settled down. I've done this several times with
similar results.

In town the car gets the same mpg whether I run the recommended octane
or 87. Seasonal/regional differences in fuel seem to make more
difference in mpg than octane rating. Last December (long highway
trips) I got 24-26 mpg on the highway outside cities vs. 22-23 when I
filled up in places where the pumps indicated a pollution-reducing
blend.

Fuel blend issues aside, on the highway the car gets slightly better
mpg with premium fuel; the mpg advantage grows with increasing speed;
I get very little benefit at 60 mph but a measurable 2 or 3 mpg better
at 70-75 mph. The harder the engine works (heavy load, mountain
driving, etc.) the more mpg premium delivers. On the other hand,
running at 60 mph is more fuel efficient than 75 mph so the most
economical way to drive is with 87 octane at slow speed. I've
recorded  as high as 27 to 28 mpg (vs. EPA 23 mpg) on the highway
using premium fuel, cruising at 75 mph under very favorable conditions
(high plains, no traffic, Mobil 1 oil). And, no, I didn't try 60 mph
while driving across the plains.

Then again, if you drive your TL as if it were a minivan, perhaps
you're in the wrong car...:-)
I've not run similar experiments with my Corvette.

YMMV