C7000-100NAS fails to perform its advertised speed

Spectrum's on site shows both v1 and v2 compatible with 400mbps speeds. 

https://www.spectrum.net/support/internet/compliant-modems-spectrum-network/

So unless they've changed the requirements, it should be able to get those speeds. 

And ISP's have changed "minimum requirements" for speeds before. For example cox changed it so speeds greater than 150mbps now require a docsis 3.1 modem. 

I haven't heard of spectrum doing that so far. 

And the "up to speeds" are just what ISP's have the capability of rating it to with their own provisioning. Not all do though as they ISP chooses what speeds to certify it to. 

so to answer

1. Yes it can handle 400mbps

2. Basically v1 to v2 can be small changes like a die shrink on a processor, update to processor, etc. 

What speeds are you getting from the C7000 hardwired into it?