Translation:The restaurant is open on Sundays.
I suppose you'd be more likely to say that 'The restaurant used to open on Sundays' ('Restaurangen öppnade på söndagar' in Swedish?)
I think my vague memories of learning about the perfekt form confused me as the 'har' kind of implies past tense to me, as well as the fact 'har öppet' just seems a bit odd literally translated to English.
I think the "has opened" could be said if it was more of an announcement, like if someone unlocked the door to a line of people outside and said "the restaurant has opened." Or if someone just wanted to sound a bit silly or different. It could also be used for the first time a restaurant opened, or if it has reopened after a long term closure "the restaurant has reopened" in that case. Overall, I wouldn't say awkward, but most likely just a different way of saying it.