Wheels are one of the biggest factors in bike performance. Bearing drag is absolutely negligible, but wind resistance and rotational inertia are very important. Rotational inertia is largely a function of rim weight, and this determines how much force is required to spin the wheels up (accelerate) or slow them down. Riding a lightweight set of wheels in stop & go traffic is a revelation, as is riding deep-dish aero wheels at sustained high speeds. The ideal wheelset is both aero, and has low rotating inertia. These goals involve contradictory compromises.

If you are riding at a level where performance really matters (i.e. - money is involved), then only tubulars will do. Speculating about 'performance clinchers' is like thinking about spoilers on a Honda Civic. Clinchers are so fundamentally performance disadvantaged, I would not spend any significant money on them. Perhaps this is what the LBS meant...