PPPoE Tunnel max speed issue

Im currently running a PPPoE Tunnel to a client who has ordered a 20Mbps,

The profile is set to 200000000 Rate Limit (rx/tx)

VIA a direct connection on the router i get results of 20Mbps Up 20Mbps Down

But through the Tunnel the results are 8Mbps Up and 20Mbps down

Also the Dude software shows the Tunnel Speed at 10Mbps

Could someone clarify for me if they have had the same issue and what the upper limits of the Mikrotik PPPoE Tunnels are

Many Thanks

Ricky