Including the boot=<path> parameter with elf2flash command above does not seem necessary - the flash file is unchanged.
As far as I can tell, the epcs bootloader code that is running from the FPGA epcs module does it all. There would be no reason
to add anything to the app image stored in the epcs chip other than some header info to tell it what chip maybe.
My confusion on compressed files was that the actual sof,pof,jic files dont change size.
If I use the map option in the file converter I can see there is a difference.
I think the sof2flash will produce compressed output only if the sof is already compressed.
The file converter produces compressed files but it only takes sof as input
elf2flash does not compress so I don't think your u-boot image is compressed.
I am referring to the cyclone (de)compression, not the epcs (de)compression only on epcs4 and up.
It is looking like there is no way for application images stored in epcs1 to take advantage of the cyclone (de)compression scheme.
It appears to be just for config data.