In your context most English speakers wouild be confused if you said that a firm had excellent standards. Standards of excellence would be thought of as the standards set by someone other than the firm itself. Excellent standards would b e thought of as standards which the firm set for itself, perhaps better or not as good as the industry.
I would think it would be the opposite of what Harry said. I think "standards of excellence" implies standards they have set for themselves, and "excellent standards" are standards set by an outside party.
Actually the more I think about it it seems that "standards of excellence" could mean standards that are necessary to be considered "excellent", whereas "excellent standards" just defines the word "standard", meaning their standards are excellent, or generally very nice.