As far as I can tell McLaren is 7kph down on the straights to Renault, which is the same gap Renault has to Mercedes/Ferrari. To me this says that Honda have improved somewhat, as at the beginning of the season the gap to Renault was larger than the gap Renault had to Mercedes/Ferrari.
It speaks well of the chassis, that the car is able to outqualify the Force Indias which were 16kph faster on the straights, Bottas was some 2 or 3kph slower so I'm guessing that's the difference the added drag from the superior Mercedes chassis makes. So if the McLaren chassis was super efficient, maybe they'd gain 3 or 4 kph down the straight(the difference between Red Bull and the factory Renault), the difference is just a lack of power. To get an extra 12kph down the straight the difference is ~60hp overall, which is more or less what others have speculated.