High performance DSP processor with better on-chip memory
That depends a bit on the constraints: are you out of code space or data space or both?

Anyway, the typical way of dealing with space problems is to use external memory in conjunction with a fast access internal cache. An alternative would be something like this http://www.ti.com/lsds/ti/processors/dsp/c6000_dsp/c66x/overview.page with a few gigs of DDR attached.

Obviously external memory is somewhat slower than internal memory, so the trick is to move data efficiently into internal memory, so that it's there when you need it. In many cases the processor will do a pretty decent job in managing this automatically, especially if it's mostly code (not data) and most MIPS consumption is in loops. 

If that's not good enough, there are certainly strategies to manually optimize code for cache efficiency as well.