15 feet isn't long, hell in the audio world 30 feet isn't long. At least not for that application. Power (watts) is a product of both CURRENT and voltage.
First, look at it like this, say for the sake of easy math we're talking about 100 watts even. It would take 10 volts AND 10 amps of current to have 100 watts in our imaginary circuit. Now I wouldn't want to try to flow 10 amps of current through a 20 gauge wire, it would get ugly. However the power supply in an amplifier turns your cars 12v system into an AC signal, runs it through a step up transformer, and then turns it back into DC for the amplifier section. Now, let's say that in our imaginary circuit that the voltage is 50volts, not 10, that would mean the current would only be 2 amps to supply 100 watts of power. The 20 gauge wire may still be a little light but for the distance of run inside a vehicle it's manageable.
Now we get into the other part of the way it works in audio. Just because an amplifier is rated as being capable of 100 watts of output does not mean that 100 watts is coming out of its outputs all the time. In fact, the only time the output gets anywhere near the full 100 watts is when the input signal is causing the amp to try to produce maximum output. In other words only when you have the volume cranked. Even then it's not holding there at maximum. Music is dynamic, it's lots of low to medium level material with some high level and occasionally maximum level material. So even at full volume 90% of a full song is low to medium level material, 7% is high level material and 3% is maximum level (depending on the music itself, how it was recorded and the quality of the recording. This is only an example).
That means that even when you are listening to your music as loud as it can safely go that your amp is only outputting in its upper power ability 10 to 20% of the time. That's why we try to tell you that reading into and worrying so much about the numbers is such a waste of time.
Trust me, I used to worry about every little number and everything years ago too. It wasn't until I spent a lot of time on here reading up on things, asking questions and talking to people that I realized that all my time and energy was focused on the wrong aspects. There are far more important, far more consequential things to worry about that make a much bigger difference than the power you're running to the speakers. That's not to say that what you're worrying about doesn't carry with it some merit but it should be lower on the scale than things like HOW the speakers are installed. Things like location, aiming, deadening, and many more items are the sum of HOW they're installed. The closer that stuff is to as good as it can get the better the overall result no matter how much power you run or things of that nature.