When I had a car(Lincoln LS) that said 91+ on the fuel filler door and in the manual, I ran it. I never had issues, per se, with running on regular but at the same time I could feel it retarding the timing(especially in terms of a seeming "hesitation" off the the line) and could measure a difference of 1-2mpg less on regular. We don't get 91 here-it's usually 92 at "budget" stations and 93 at name brand stations. I understand this is a regional thing, and 91 is premium in some parts of the country.
My MKZ says 87, and I run 87 in it. I've run a few tanks of premium back to back in it in the past, and could not see any difference in power or economy.
The MG is a bit of a different story. I tend to like how it runs with the timing a bit more advanced than normally specified(a lot of sources will say 32º BTDC max advance no vac-I usually run it 35º or so). I'm set up with a head/piston combination that pushes the C/R higher than stock(9.5:1 vs. 8.8:1) which, without a knock sensor, generally dictates 89 as a bit of a safety factor. I just go with 92/93 and enjoy the benefits of the timing a bit more advanced.
BTW, when I first started paying attention to premium in '08 or so, when I got the LS, typically premium was 20-25¢ more than regular. Now it's 50-70¢ at most stations.