Generally adding timing helps spool up, however you can go too far both ways. Add too much timing and you will hurt spool and raise the risk of melting a piston. When timing is correct, the engine will run its coolest, and have the best power as it doesnt labor as hard. Diesels are a bit different in the manor of timing as changing timing is done to tune in the turbo more so then it is the engine. The correct amount of timing for the cylinders efficiency might not be the same as the correct amount of timing for the best turbo spool. Adding timing enhances how much fuel is burnt. The more fuel you can burn thru the power stroke the more gases that exit the exhaust port which in turn helps get the wheels turning in that turbo..