You should get aggregate bandwidth, not just load balancing.
Not really a DSL guy, and don't have access to that connection type to play with.
You might want to tweak your MTU settings on the interfaces.. But the pfsense board would be the place to ask - there are a few people here that are using pfsense - I have converted quite a few over I think
But not sure any of them would be doing MLPPP.
You using or going to be using 2.01 or 2.1 betas? I have been running the 2.1 line since first came out and been very stable.. But they are sticklers about the details before they release. So could be some time yet until 2.1 is released for production. But more than stable enough for even production work if you ask me. And per the board there are quite a few people using it in production setups not just lab/home/play/etc