said by IowaCowboy:said by n_w95482:343 without factoring in overhead, 304 with. It mentions it right below where you looked 
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The Zoom 5341J also goes up to 343 with its 8x4 channel bonding.
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www.zoomtel.com/products/cable_overview.htmlI think they should make the 305 available on the coax plant as well but for a lower monthly price and the $1.99 change of service fee if we pick the modem up at the office.
I also think the modulation also affects the speed. Our area is 256 QAM downstream and 64 QAM upstream.
Edit: I did not see 304 mentioned but a properly engineered HFC plant could achieve 305 by splitting larger nodes, ditching analog TV, declaring all D2 modems end of life (since D3 manages network resources better), adopting switched digital video, eliminate as much ingress as possible.
I also heard that DOCSIS 3.1 is possibly in the cards.
Overhead is overhead, there's no getting around it. It's like saying that you get 54 Mbps on 802.11g wireless, or 480 Mbps with USB 2.0. It just isn't going to happen. They quote the theoretical max because it is easy to calculate. Real-world numbers will always vary, and will always be below the theoretical max. 305 Mbps downstream on 8 channels isn't going to happen. I don't expect Comcast to attempt it via DOCSIS until 16 or 24-channel bonding is around.
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