I assume you are doing NAT to get out to the internet?
I have a couple of RB2011s and by using NAT they max-out at 150-200mbit (also I have a bit complicated configuration with vlans, bonding etc - but really simple firewall rules < 10rules).
The bottleneck is the CPU itself since it cannot handle more than that. NAT takes up a lot of resources at such high speeds.
Also set your PPPoE Client interface MTU to 1492 as dadoremix correctly pointed out.
TCP-MSS should be set automatically if you are using ‘default’ or ‘default-encryption’ profiles on the pppoe interface (unless you have changed them).
No need to create your own rules for that. The automatically created rules by Mikrotik work just fine. I haven’t had an issue with those since version 4? I don’t even remember how far back 
There’s also the solution of fasttrack on the latest versions of Mikrotik but I haven’t tested that myself. You should also check that out.
Here’s more info about fasttrack http://forum.mikrotik.com/t/fasttrack-new-feature-in-6-29/87517/1