Hmm what do the 58xx report their name as? Cypress? It sounds like cgminer should default them to worksize 256 instead of 128. Would that be a fair assessment?
Yeah, it's Cypress.
The best worksize really depends on the user correctly setting the ram clock; on the default (1GHz), it's best 64 or 128 with 4 vectors.
Gonna link again the graphic I like so much

(from phatk thread)

Yes I remember this old graph. It would appear to me that cgminer's defaults of v2 w128 are the most robust for all tunings unless the user goes out of his way to tweak things manually, in which case he'll be choosing his own settings, thanks.