Does a 1500W blender produce a less viscous smoothie than a 400W blender?

The measure you should use for making the smoothest purées is RPM, not wattage. You could put a blender jar on a 4500 Watt floor standing mixer and it wouldn't do anything more than roughly chop some berries (or car tires with that kind of torque.) Assuming the canister and blade designs are good, and it's not drastically underpowered for its speed (something I haven't seen before,) higher RPM blenders will pulverize the contents of the jar much more smoothly than lower RPM blenders. If you're looking for a general puréeing tool, a high-speed blender is what you need; if you're just looking to make juice, a cheap juicer will give you more bang for your buck, and you won't have to strain particularly fibrous juices.

Edit to address your updated question: Irritatingly, the difference between an expensive professional/prosumer high-speed blender and a reasonably priced standard consumer blender is significant. A standard consumer blender, within a reasonable amount of time, would make a slightly pulpy mixture out of soft fruits. In the same time, a high-speed blender could make a velvet-smooth mixture from raw carrots. Your mixture will be much smoother— so much so that some people find the smoothness unpleasant. (As an aside, I'm not sure viscosity is the best word to use... The solid matter will still be there, it will just be in significantly smaller pieces. Uniformity or smoothness I think are better descriptors.)

Edit Again: As KevinNowaczyk pointed out in the comments, "Sheer" is the measurement of thickness in purees.