Max throughput of SOPC DMA?

You should be able to hit around 97% (max) efficiency using a SGDMA depending on the access pattern. I recommend using the HP2 or Uniphy controllers since they don't need burst accesses to be efficient. If you access them sequentially then internally they'll form optimal offchip bursts for you.

If you want to continue using bursting I highly recommend that you match the burst counts of the DMAs to the memory to avoid burst adaptation. You might also run into burst wrapping efficiency problems as well since the DMAs don't line themselves up on burst boundaries except this one: http://www.altera.com/support/examples/nios2/exm-modular-scatter-gather-dma.html?gsa_pos=1&wt.oss_r=1&wt.oss=sgdma