Compression definitely isn't the only factor, ESPECIALLY today. My 12.5:1 car would be some 100LL sucking dragster in the 1960's (Well, assuming JUST the compression ratio); but it's an 87 octane 4-door econo-commuter today. Actually that's becoming more and more common. Very high compression ratios on small engines with variable ignition, valve, and fuel timing (my car has twin independent variable camshafts) tuned to run on the lower octane fuels. It does take advantage of higher octane fuel with a noticeable and dyno-readable boost in horsepower, but runs just fine at 87 and actually is more fuel efficient (likely because it just pulls so much timing).
The thing about octane ratings, similar to other ratings, isn't that "the moment 87 touches your bike it will instantly explode and your Dog will get cancer". It's that in just the right conditions, damaging detonation could occur. Those conditions could be accelerating up a hill in a hot day, pulling away from a stop sign or a light after sitting long enough for the engine to get hot, or some other situation where the engine is primed for knocking and pinging, and would need higher octane fuel to prevent that.
The 900 has the exact same 9.5:1 compression ratio and happily runs 87 all day long and that's all the manual says it needs. But it runs cooler, doesn't heat up at a stop, and probably doesn't have as much ignition advance as the 1700.
LOTS of factors can create a condition requiring high octane fuel. Ignition advance, compression, heat, engine load, the presence of carbon buildup in the engine, etc.
IMHO; use the minimum the manual recommends and not any less; but using more probably won't give you an advantage. So use at least 91. Some get along just fine with 87 in the 1700 engine, YMMV.
One thing missing from our bikes that almost every car has is an anti-knock sensor. If we had those, we wouldn't have to worry about it. When it detected any sort of knocking or pinging, it would pull timing. If, after pulling timing it still detected it, it would throw an error code. Alerting us to correct the condition causing it or use higher octane fuel. That's how it works in your car and that's why on a car, a Dyno can tell if you're running premium (inversely, higher octane means more aggressive timing so more power is made), whereas there's little difference on a bike on a dyno (assuming EVERYTHING ELSE is exactly the same. Though TUNING a bike to run higher octane fuel can definitely improve performance). Note that knocking/pinging isn't always audble to the rider and causes slow, gradual damage to engine components. As we have a lot of 1700 owners who run 87 and some who have run it for a LONG time (though, one particular case,
@RACNRAY, does have a Dyno at his disposal to tune things) and nobody has reported a hole in their piston; I doubt it'll be an issue. But it's food for thought, that just because you can't hear it, doesn't mean it isn't there.