With the exceptions of Turnbull and Watt (so far anyway), who was the last player to leave us for a rival SPFL team that has made a lasting impact for their club or bettered themselves?
You look at St Mirren with Tait and Dunne, plus Main and Erwin via other clubs, and you'd be hard pushed to say any of them are making the Buddies better. Main's time at Aberdeen was far less impactful than his time with us. Going by Wikipedia, he scored more for The Well than Dons and St Mirren combined so far.
Would you need to go as far back as McDonald going to Celtic?