The new HG659b is doing the same result.
Down stream can not go over 300mbps, up stream is always maxed out.
Not sure if there is a new version of HG659b now, since the two I have seems identical, except the old one come in a green box says "Business Gateway", the new one in a orange box says "Spark HG659b Modem". So I emailed back Spark and said there is no need to post me another modem.