according to the mikrotik wiki the MTU (maximum packet size without MAC header) is 1580.
That is enough for about everything you need.

(this is the first result when googling for RB850Gx2, so I just wanted to fix that)

also there is a bug with encrypted VPN connections since OS version 6.8 and not fixed until this moment. I can confirm that. It causes connection drops. For example you are able to connect with WinBox over VPN, but after a few seconds it disconnects. Also opening Webfig or loading the graphing-imag...

How about http://web.nvd.nist.gov/view/vuln/detai ... -2014-0224 ? As far as I can see, this allows man in the middle attacks and allows to steal the session, when you use the RouterOS WebConfig, maybe also a SSH connection, but not sure about that.

what version of Ubiquiti AirOS are you using? we use the 5.5B11 on the CPEs and they are able to connect to our RB750GL's using PPPoE for days without problems