The SB6141 supports 8x4 channels. If your cable ISP supports higher bandwidth within each channel, it can in fact go up to 343 Mbps.
Most cable ISPs don't go that route however due to line quality issues. Instead they just give it more channels, which the SB6141 does not support. Here's the list of modems TWC supports and the speeds you'll get with them on their network.http://www.timewarnercable.com/content/dam/residential/pdfs/support/internet/twc-compatible-modems.pdf