What RPMS are you turning? Add extra wight to boat by adding stuff? Been very humid, mine loses RPMS and speed till the weather is cooler. Lower unit was apart or just change impeller? Your plug gap should be fine if they are NGK. If your compression is good, no idling issues or missing at WOT or any range it may be hard to diagnose. I'd fix the ding in prop either way.
My 97 150 turns a 23 p prop at 5500-to5600 in cooler weather and 5200-5400 in the humid stuff, mph varies from 55-60 depending on load and weather for me.