It's not that straight forward, we've had many people ask this question! It is very easy to waste watts, it's all about getting the highest average speed.
For example, if you push really hard at high speed, you expend watts overcoming wind resistance, which squares with speed! If you push really hard at low speeds on steep climbs there is little wind resistance, and gravity is linear, not exponential! Therefore you get more speed for the extra watts.
There's a common joke about it, power up the hills and freewheel down them, it works much better than the other way around.
There is really no reason why you should expect to go faster just because your watts are higher, or your watts/kg, it's not how much power you have, it's what you do with it!
Geoff