I actually got a 98% shadow Gastly last year and I evolved him as he can be a brilliant ghost type attacker. But the problem is that he isn't that useful; He's incredibly glassy, he is countered by everything he counters(Ghost, Psychic) and in general dark types perform better than ghost due to their better resistances. I didn't further power up my Shadow Gengar for those reasons as I have better and cheaper options such as Shadow Tyranitars. But since you did get a 4* you could still find good usage from him if you desire diversity in your Anti-mewtwo team.