If anything the snorkel would improve economy by a small amount, because by taking air from outside the engine bay, the air is cooler and thus more dense - it's like turning up the turbocharger output by a small percentage.
It's unlikely to be aerodynamic drag, and if installed correctly it won't be a restriction issue because the snorkel and the air path into the airbox is much larger than the apertures in the air filter (specifically, the bends that you can see on the outside heading to the bottom of the box where the baffles are). However, if you suspect it's starving a little, it would pay to inspect the air path - easy to do with an inspection camera.
If not the air restriction, it could be a variation in any of the following: local air pressure, humidity, fuel quality (especially noticeable after a tanker has delivered and "stirred the tanks"), even a slight change in temperament - I notice I'm a little heavier on the foot if I have to get somewhere in a hurry.
