The xax speed for Premier internet service is 150 mbps which an 8-channel modem should be able to handle. Question is, will cox try to deliver the speed over more than 8 channels at once? Could I possibly get better, more consistent, speed with a 16 channel modem over a 8 channel modem?
Probably. The question is how much, which can only be determined by trying it. What model modem do you have now, and what are it's signal levels, which are usually on 192.168.100.1 webpage.
I don't have the service yet. I'm getting it installed next week. I'm looking at buying the NETGEAR CM400 which is only a 8 channel device.
Impossible to say then. The CM400 is approved up to Premier according to Cox's site, however that doesn't take into account congestion in your area, which 16x4 may work better. If it's a matter of money, the CM400 will work, but the CM500 or SB6183 would give you some growing room.
I would get a 16 channel modem at a minimum. I've been active on the Cox forum on dslreports.com and most Cox service areas are 16, even 32 channels (Seems popular in Arizona).
You'll consistently get your advertised speeds by spreading load across more channels. If a couple channels are congested due to other modems on the same node, your other channels can pick up the slack.
I recommend you get a modem that employs a Broadcom chipset due to the publicly acknowledged latency/jitter issues with Intel's Puma 6 chipset used by a number of modems. Some Cox supported 16x4 modems that use Broadcom chipset include Arris SB6183, Zoom 5370, Motorola MB7420, or Netgear CM500/CM600. All are 16x4 modems except CM600 is 24x8.
I think Arris SB6183 is a quality modem that is well supported. I've seen refurbished units as cheap as $50 on slickdeals.net (via Newegg) or new ones are at my local Costco for $75.
Also Cox supports Netgear CM1000 which is a DOCSIS 3.1 modem for when they offer gigabit speeds in the next couple years - but its really expensive $180. It uses a Broadcom chipset and is the only DOCSIS 3.1 modem available for purchase right now.
Just putting it out there as an option.
