I'm running a Vortex 16 up front and a stocker in the back. The guy hanging onto my bike right now tells me it rides so much better, I will take his word for it (changed it out over the winter and left the country as it was still snowing).
There are some things to consider prior to altering the gearing.
A front sprocket with fewer teeth works the chain harder and robs power, as the chain has to curl more to match the smaller diameter.
One thing to watch is clearance between the swingarm and chain. Smaller sprockets (Front or rear) can result in the chain laying on the swingarm, and actually working the suspension when power is applied. With high-horsepower machines, chain pull and its effect can be altered using sprocket selection. Anti-squat torque-which extends the rear suspension under power-is determined by swingarm angle and the distance between the swingarm pivot and top chain run. Even if the chain doesn't touch the swingarm, running it close to the pivot will enhance the anti-squat torque, much in the same way raising the swingarm pivot does.
the smaller front robs power? I can see what you're getting at but does anybody have a dyno between a 16t front 40t rear vs a 17t front and a 43t rear vs a stock setup? I'd have to see it to really believe it would be anything worth even noting.