Internet plan speeds question

Hello, I currently pay for the 300 Mbps internet plan but I have never seen it go above 90 Mbps. But that's fine, 90 Mbps is plenty fast enough for me. My question is, if I downgrade to the 100 Mbps plan, will I continue to receive 90 Mbps? Or will I always get some lower fraction of what I'm paying for?

Before you ask, I have a DOCSIS 3 modem that should be able to provide speeds greater than 300 Mbps. I have talked to Cox and they say everything is fine or they try to rent me one of their modems.

Thanks guys!