Heat range 7 is too cold for a stockish engine. Factory spec is 5. And for a stock engine, keep the wider gap, maybe close down to 1mm. And put the Splitfires back in.
15W-50 oil is too "heavy". Factory spec is 7.5-30. The first part of the description, use 0, 5, 10. The second part, use 30, certainly nothing more than a 40. eg 5W-30, 0W-40.
Have you checked the ignition timing is correct? Retarded timing will certainly display symptoms of poor power delivery. Are you running the factory air box, or do you have an unshielded pod filter?