Nope, does the same damage for Shadow as it does for Disc and Holy - it's a mana cooldown for all three specs.
Also, to test the claim that Stampede does less than Shadowfiend - I jumped on my 498 item level hunter (versus 550 item level priest tests) and even in crap gear Stampede outdamaged Shadowfiend, so Stampede probably does at least double Shadowfiends damage in 550 - more importantly though - Stampede isn't the only damage cooldown hunters have - they have a whole retinue of such buttons: and Stampede's an incredible source of spell pusback and clusterf--kery.