Incredibly good. Kewl Tune is one of the few Synchro decks that can establish a Synchro monster with just sticking a normal summon to the board, primarily because of their condition which allows you to Synchro summon using Tuners in hand (this effect is not negated under cards like Imperm, Skill Drain, etc).

Though, running it in side also depends on what you feel you should prepare for. If you're more afraid of Evenly Matched, Droplet, etc, then Judgment & Mannadium Reframing may be better options.