Without a new engine I can't see them getting near the V6 in the Alfa Giulia in terms of output. The new QP S has the same V6 as the Ghibli S so I don't think they have a comparable engine yet. They could turn up the wick, as they've done in the Levante, and squeeze another 20hp out of it without impacting drivability but upping the boost to get to the 500+ mark may come with increasingly laggy performance. So I don't see a facelift getting much more power unless they decide to rebadge the Ferrari/Alfa V6. I can't see them putting the V8 in the Ghibli either, though I'd look to swap mine if they did
I think we'll have to wait for the post Levante cars for a refresh of the V6 and then we'll start to get more power in the saloons. Though if the V6 is producing 520hp what would they have to bump the V8 to from its current 530hp?