question about N450 and Time Warner Cable

I signed up for TWC's 200/20 internet plan and I my speeds hadn't changed at all (was on the 50/5 plan before) so I contacted TWC customer service about it.  They basically said the N450 I was using maxed out at 50 down and that I had to get a new modem. Is this right? Doesn't seem right unless I'm missing something.