Is it the posh, premium supermarkets?
No. It's a mid-market franchise. They differ a lot depending on how big the premises are, how good the owner is. Plus, it's virtually an MLM pyramid scheme with different tiers of profit margin, something like 'uplines' etc., so they (and their pricing) differ a lot. Some of them make their profit on vodka, beer, cigs and peanuts, others have more fresh food.
Oh and do Polish supermarkets offer locally grown produce?
Yes, but not only. Local is cheaper, however if you want tomatoes in January they won't be Polish.