In race pace, I don't think they are miles behind. If you consider that this is a circuit that rewards straight line speed, and that Redbull were strong on the straights, yet could still manage tyres well (with Ricciardo). He did his best lap on the final lap of the race.
In raw one lap speed, they won't match Merc all season, but in race pace (when all cars are bottlenecked by the tyres) they seem pretty close. Probably not close enough to win on pure pace alone, but close enough to hang in there and be in the hunt. Monza will be tough but Singapore afterwards will be interesting. If you could say that there would be this result, after the winter testing you'd have never believed it. Hungary you could say that it was due to the nature of the track, but Spa was a Merc track. The problem is, there hasn't been a good straight fight between Nico and Lewis without problems. Nico's race pace has seemed a bit off. And Lewis didn't seem that quick either at the start before his hit (hence, Nico attacking him). If they have a clean race with no traffic, they'll probably be dominant, but when things don't go well.. they are a lot more vulnerable now, than they were in the first 8 or so races of the season.