150 miles is well past break-in stage. Switch to 32:1, then clean the air filter, or preferably upgrade it to one that isnt restrictive (won't effectively choke the engine at high RPM).
Having the needle dropped one notch below the middle won't do any damage. Actually, it is the most common needle setting.
You might want to get a smaller jet but I would wait until you have a great air filter first so you can see what that does.