re: DOCSIS 3.0 is the tits (cable modem upgrade)

Question

Right now I have an old (3-5 year old) modem connected to a new router. When i first setup the internet in my new house, the signal wasn't strong enough to reach my bedroom. I had several walls and a brick wall interfering with the signal. To combat this, i moved my modem and router to a central bedroom, and i had to buy an extender and put it in the living room. I was getting decent signal. Well my sister-in law and her husband are staying with us until their house gets built. I've noticed that my internet signal has deteriorated, probably because of more devices using the signal. Right now we have my phone, my xbox, my wife's phone, her ipad, my sister in laws phone, my brother in laws phone, and his ipad.

So they are all sucking signal thus reducing signal for my internet. I recently did a test and my download speed was 25 mbps. I'm sure it is slower now.

Would buying a DOCSIS 3.0 modem increase my signal strength, or am I limited by the Cox service? Not sure what package i have for internet, but it costs me about $60/month.