i've known horses with fairly pronounced stringhalt that were winning Open Point-to-Points, and eventing to a decent level. one eventer was I/A but her stringhalt only really showed up when she'd been standing still for a while, oddly.
My friend's horse had the most severe stringhalt that the local veterinary school had ever seen...it didn't stop him winning just about everything on the local level showjumping, and he lived til mid twenties.
S