1.2 Gbps service only seeing 400 Mbps speeds

I've worked with support a number of times and nothing I have tried has made a difference. Here are the details:

Upgraded my modem to a Arris T25 and subsequently upgraded from 300 Mbps to 1.2 Gbps service (asked for 900 Mbps but was upsold). I understand without bonding there are only 1 Gbps ethernet ports on that modem so I would never get the full bandwidth to a single device but I would expect something in the 750 - 850 Mbps range at the very least. 

However, after upgrading I can get speed no higher than 400 - 430 Mbps. I've tried directly connecting to the modem (bypassing the router), I've removed every and all splitters from the run including in the attic, I've even installed an amp (as my power levels were routinely around -8/-12 db which I wanted to get more in the +8/-8 range, which are now hovering around 0 db). Nothing I have done has changed that number whatsoever, its like the same speed test every time. I would think if it were a bad connection in the run or a bad signal from the PoE then I would see some variation in speeds as I run different tests and different setups.

Support has validated the service upgrade on their end and all diagnostics that they run come back fine. I'm pretty frustrated at this point. My next step is to drag the modem outside and connect directly to the drop to prove it is nothing with the internal wiring as that is the typical goto but its been raining today. Any other ideas that can be tried? I'm not looking forward to my 7th call to Comcast over the last 3 days.