It looks like most of the DesignStart is oriented at the M0 and M3, but the M4 DSP instructions are of interest to me. I don't need floating point support, but having it wouldn't hurt.
I'd also like to be able to develop peripherals (M4 and M33) and custom instructions (M33 to start) on the same FPGA, but I don't seem to be able to find the info on how to do that.
You're correct that DesignStart is focused more on Cortex-M0 and Cortex-M3. However, there are some ready made Cortex-M4 and Cortex-M33 FPGA images for the MPS2+ platform, which are available from the following page:
How can we modify these images?