Hello,
To answer your first question in short, yes RUTX10 capable of NATing any speed.
Regarding VPN performance, I just did a test with LT2P via IPsec configuration and I experienced a 10% loss of speed. Keep in mind that every VPN speed loss depends on the protocol you choose and encryption methods. More secure VPN connections tend to have higher losses (for example PPTP is not secure at all and use only simple encryption, but performs as fastest VPN)
Regards.