Avalon multiple interfaces performance

Designing a system with two busses can make your system go faster. You just need to balance it well so that both are used, but yes it should increase the througput, as if each master access a different slave at any given time, they will do it in parallel.

Just be sure to use the correct master when needing to access a slave.

There are other ways of optimizing an Avalon memory mapped bus usage, especially pipelinined transfers. With pipelining you can send several read or write requests without waiting for the result, and with systems that have a bit of latency it is a huge improvement. IIRC with the recent memory controllers it is as efficient as bursts and maybe easier to implement.