As for the noise, I've definitely noticed that folks around here whose outboards are newer than my 1990 2-stroke, are also significantly quieter, and when motoring to and fro at low throttle, produce a sound that could almost be described as "purring".
I don't know about stress on the motor but your guess sounds plausible. Clearly there's such a thing as too much, but it may be that the throttle cannot be opened wide enough to get to that level. However, I understand that running the motor at lower throttle is more fuel efficient. Also, does your motor have an unregulated charging circuit? If so, running the motor at full throttle for long periods of time can be hazardous to your batteries.
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27