I'm not sure whether you're recommending flying the performance chart speeds or the recommended speeds? They are different.
Granted. If one doesn't have performance data, then there remains the problem of justifying the takeoff/landing post mishap. I am comfortable with both approaches but I have an appropriate engineering background to do some approximate sums to keep myself sweet.
Cessna seems to have recognised that most of the time, the 172 will be flown off more than adequate runways.
Aye, there's the rub. At what point does one perceive a problem ?
Cessna also seem to assume that pilots are smart enough to figure out when runway length might be an issue
One would hope that pilots are appropriately conservative so that such would be the case.
What temperature do you use for performance calculations when operating into a location without a forecast?
At present, the declared data is fine. Also, the BOM site data provides quite useful information. Apply whatever conservatively and, where feasible, carry an alternate to somewhere with forecast data. I really don't see a problem with this one.
(Guess it's not all that much different in principle to detailed software development work.)