The turnaround time won´t be much different. More seats mean more seats to clean, more food to restock, more trash to take out.
Sure a FR or U2 like operation could probably squeeze a few minutes out of the process, but I doubt they are seen as operators for the plane. And once you get premium class seats and catering in the plane, restocking and cleaning those alone will eat up any advantage.
From the point a narrowbody plane connects to the airbridge, it takes normally 20-30 minutes for the boarding to start for the next flight. This includes unloading anything below deck, refueling, recycling, cleaning, catering, potentially a crew change. If the plane has 20-30% more seats but 2 aisles, I don't see anything there why it would take longer, also nothing why it would be considerably quicker. At least the twin-aisle will not make it more difficult, and if they have to add a handful of people for the cleaning crew for 10 minutes, I doubt it costs a lot.
But seriously, from the point when boarding starts, in a full 320/737, it takes 30-60 minutes until all passengers have found their seats. And that again is due to the fact, that each one of those 200 people will have to queue in the airbridge and wait for their turn to enter the plane, to lift their luggage in the overhead bin, or ask someone to lift it for them, take off their jacket, quadruple-check the seat number, then ask someone to change their seat with someone because they want to seat somewhere else, get in their seat, fasten their seatbelt, unfasten the seatbelt, get off the seat and let someone else go to the window, re-seat, re-fasten, re-member that something was forgotten in the luggage, unfasten, get up, lift down luggage, fix the issue, lift the luggage back up, cram the luggage because it longer fits, test that the overhead bin lid closes - it doesn't - ask the FA to help - it really doesn't fit - here you can put your bag here in the back, excuse me sir, coming through, sorry, pardon, myfaultmyfault, lift the bag in the empty bin (which now limits the other people to fit their's), make their way back to their seat, wait for passengers coming on, navigate, finally sit down, and then remember that they also wanted to use the bathroom before takeoff. This
is the time that can be cut in half with 2 aisles. In other words down to 15-30mins. And that does make a huge difference, no matter how much the Airbus fanboys try to argue against it.
That of course does not
have an impact on the fact that there is always some schmuck who is late for boarding, or connecting passengers, or delays with the luggage, or delays given by air traffic control. But even then, the biggest single factor impacting the turnaround time on average, is the boarding procedure.