Arris G36 - runaround with call center, tech support and Arris warranty support

I purchased the Arris G36 as a bring my own device to Xfinity 1.2GBPS to a pretty hefty cost.  I purchased it at Best Buy and they have a 15 day return window and it was purchased on 10/09 so that is out of the question (I tried to return on 10/27 and it was rejected).  So lets get down to the issues here:

I am not getting anywhere close to my advertised speeds.  When doing a speedtest, wired with a desktop that has a Realtek RTL8111H that has 10/100/1000 mbps max speed I am only getting at max 270 mbps.  I called a tech out first 2 weeks ago to which he couldn't find and issue so he hooked up an Xfinity device and voila I was getting 700 mbps.  That prompted me to return the G36 for a different device to which I subsequently found out I couldn't do.

So I brought it back home, reconnected it and reactivated it and called support for troubleshooting.  They couldn't find anything on the phone so they sent another tech out this past week.  This tech found some noise in the line at the street where I was split with some neighbors so he got me segregated from that noise and the downstream power levels increased by about 4 dbmv on average and provided a bit better quality signal apparently.  That still didn't change anything with the G36 connection though.  So again, he hooked up the Xfinity modem and activated it and voila now I was getting 960 mbps.  He left that connected and my rage fueled even more because at the direction of Xfinity I purchased a $350 device to bring (that I can't return) that is compatible with the service but isn't getting what it is supposed to.

So of course, what did I do after he left?  I connected the G36 back and I contacted customer service about the connection issues.  They directed me this time to contact the manufacturer about the issue because everything is looking green on the end of Xfinity.  So I called over to Arris support at 1-877-466-8646 to troubleshoot the issue.  When I discussed the issue with them we reviewed some key levels on the modem itself.  Here are those current results:

The upstream flow was this:

Upstream QAM

What I have come to realize is that one of the channels seems to have continued uncorrectable codewords, the one with over 1M continues to grow until it was like this morning at over 40 million before a power cycle:

When talking with Arris, they will not accept any issues are present until the upstream power level is between the range of 45-51 dbmv. 

So with this information I called for my third house call today to have a tech come look at this again but I feel it is going to be the vicious loop of "The line looks good (to Xfinity standards which is below the manufacturers standards), it is customer owned device and best they can do is hook up a $15/month modem to get the subscribed speeds.

So unless XFinity is going to give me a $350 credit to my service we won't be switching back to an XFinity owned device and renting.  What am I supposed to do here?  I want to at least be close to the speeds I am paying for, I am stuck with a modem that every tech so far has refused to work on and the manufacturer states XFinity has to get the speeds within range but XFinity says the speeds are within good limits.  I purchased this device at the behest of compatibility with the service I am paying for that XFinity stated.

Make the loop end, but how??