I have had success in my testing to run DMA datalogging on my Evo IX.
The usual MUT protocol only runs at 150-180 samples per second, and they are of limited resolution because they are only bytes whereas the internal ECU variables are 2 bytes. In a low gear logging many items you can get a fairly rough data collection. At 6000 RPM logging a typical set of items you only get a row of data for 10 engine revolutions. Furthermore, the communications are a little slow for realtime mapping - writing an ignition map to RAM was taking about 6 seconds. I was able to quadruple the speed by changing the comms routines, but it was taxing the CPU when trying to run the engine by going any faster, so DMA was clearly the way forward.
By increasing the baud rate to 62500 and using the SH2 DMA channel number 3 working the serial port 0 I have a vastly quicker datalogging. I will be converting my RAM read/write map routines to DMA to make the transfers within 0.1 seconds.
If Subaru owners (using 32 bit SH705x series MCU) want to implement something similar I could help with the details of the DMA transfer. However, a good bit of the work required is disassembling the serial comms the ECU uses and understanding all the flags it sets etc so that you can take over the serial port for the DMA and then transfer control back gracefully to MUT or SSM. It has taken a lot of disassembly on the Evo to get to this point, and I've been thinking about using DMA for about 8 months now.
How fast can SSM go at present?