Out of curiosity, how much of the 138K LUTs (as well as other resources like BRAM) are in use here? I wonder if there's much room to add fancy peripherals, or perhaps "growing" the CPU to achieve better IPC.

It currently uses 44% of the LUTs and 59% of the BRAMs (out of 340 × 2 KB blocks). The chip itself is fairly large and inexpensive, though performance leans toward the lower side.