"寿司屋" means just a "sushi restaurant". I've heard "お寿司屋さん" spoken a few times. As far as I know, only a person's name can be suffixed with "さん". So, is "お寿司屋さん" a personification?
「お寿司屋｛すしや｝さん」 is composed of 「お」,「寿司｛すし｝」,「屋｛や｝」 and 「さん」.
Here, 「屋｛や｝」 is affixed to the name of some merchandise (「寿司｛すし｝」「魚｛さかな｝」「石｛いし｝」 etc.) or service （「クリーニング」「修理｛しゅうり｝」 etc.） to mean:
A shop selling the merchandise or providing the service or all those shops as a whole
The owner of a shop selling the merchandise or providing the service
When 「…屋｛や｝」 refers to a shop, adding 「さん」 to it is in effect treating it as a person (although I feel that native speakers may not have a very personified image in their mind when using it). That shows slight intimacy (I guess intimacy can mean very close relationship in English but that's not the case here) with and a certain degree of respect (since 「さん」 is in itself a suffix for that) to the shop. It's generally colloquial and used a little bit more often by female speakers.
The 「お」 at the very beginning is a 丁寧語｛ていねいご｝ adding politeness, towards the one that is talked to, to the whole expression.
「さん」 can be seen to be directly suffixed to company names as well. But according to this page, its level of acceptance varies a lot among native speakers.
ーさん can be used as a courtesy title to address or refer to others with certain occupations. eg:
The addition of the お prefix to your example is an example of お being used for politeness, as opposed to respecting the position of the listener.
Reference: "Japanese for all occasions" by Taeko Kamiya, p13 & 17
In my experience, when used with a shop or restaurant it tends to be a small business. If we take 肉屋 as an example, in Japanese it translates as "the meat shop", which is slightly different from the English term "The butchers (shop)", which refers to tradesman rather than his wares. When I hear "肉屋さん" I associate with either the shop or the butcher(s) himself (themselves) depending on the context.
Also, although I have not seen this written down anywhere, to me this use of さん is somewhat similar to the way we might refer to a restaurant in the English statement "They have vegetarian dishes on the menu."