R6700v3 New Firmware Slow Speeds

I also have a 6700v3 which was getting 350Mbps with firmware version V1.0.3.66_10.0.50. As soon as I upgraded to V1.0.4.anything the speed went to about 170Mbps at most. I downgraded back to V1.0.3.66_10.0.50 and my speed went back to 350Mbps. I've tried everything I can possibly think of to fix the issue, but there is nothing I can do to make any firmware version past V1.0.3.66_10.0.50 work at anything near full speed. I'm really feeling trapped by Netgear since my only choices are to operate on an insecure firmware version and get full speed or use a "secure" version and have my speed cut in half or more. I guess I do have a third option: throw the router out and buy a different brand. I don't like any of those options.

I am a network administrator of 20 years and have deep experience in routing, switching, firewalls, etc. This kind of issue especially bugs me because I have no recourse. Even if I paid for support with Netgear, I wouldn't get any help. The firmware has broken the router fundamentally and the only resolution is for Netgear's dev team to concentrate their effort on my specific model, which won't happen. I'm not hopeful at all for a fix.