Eric,
I think your 10-14, for winter, VT driving, is right in the normal range. I personally think you're wasting money using 93 (especially in colder months)
Even more so since your '95 has a distributor and no knock sensor. You will doubtfully see ANY change by going to 91, possibly even 89. I have driven my 110 for years on 89, and when I recently tore it down, after puting a 4.2 in, found that my top end was just as clean as any other engine I have worked on. Gems ECU doesn't have the flexibility in timing adjustments, that's why knock sensor trips on low (87) octane and shows a 'chk' eng' light, but you can get your timing smack on with the distributor, between advanced enough to prevent condensing VOCs, and too far advanced, causing low end knock.
I hear people saying numbers like you suggest too, and frankly, in US efi Rover v8s, I am VERY skeptical of those high end numbers(anything above 18mpg). (Watch this bring out a bunch of, um, how should I say this, people (yeah, that's it, I'll call them people) insisting on their accurite calculations. All I can say is, eventually, they' ll have to head back uphill.