Cant use personal modem, forced to rent

I recently purchased a CM500V modem but have been unable to use it with Xfinity due to problems with thier configuration file for that modem at the 200Mbs speed tier.

It works fine with 100Mbs setting, is rated for 300Mbs (a tech said he runs that tier with that modem fine as well), but when applying 200Mbs configuration the modem drops below 70Mbps which at least one of the three techs I spoke with said is a "default config". 

The phone support escalated this twice, but said they have no way of knowing if/when it will be fixed as they dont have communication channels with tier 2 support. Thats pretty awful in itself, but it means I have to spend ~2 hours swapping gear every month to determine there havent been any changes. Getting tired of that, does anyone know if there is way to address this directly? Phone support hasnt been useful and none one can escalate to tier 2 or trusts that they are even looking at the problem.