The stock starter on the CVO 103" and 110" engines is a higher output 1.4 kW starter, but the starter drive clutch is nothing special and plenty of folks (including yours truly) have had them fail. IF your complete starter was replaced, it was probably unnecessary since the actual starter motor is very reliable (it's made by Nippondenso and it's used in automotive applications, not just on Harley's). I'd suggest replacing just the one way clutch, which you can pick up for about $40 including shipping on ebay versus the $100+ for the Harley kit. While you have the outer primary off to remove the starter, closely inspect the ring gear on the clutch for damaged teeth.
I have no idea what that mechanic has in mind to "fix" your problem for $150, but he is wrong when he says the starter isn't powerful enough for your engine. Maybe he plans to replace the one way clutch and he's charging $100 labor plus the part? I'd suggest you find out exactly what he is talking about before having him do anything.
BTW, is the engine modified or basically stock? If cranking compression is significantly higher than stock due to modifications, then you may need to think about compression releases. Just a thought.
Jerry