I just upgraded to 256 kB. Two hours of soldering :) The cold startup time is now 8 seconds. Not really bad, but annoying.
Since we are creating a IEC cartridge, I thought of putting it at $2000 (so it would be the first cartridge to run) and include our own initialization routine. It would perform all normal initializations, but with only a rudimentary memory check to speed up the startup. Then it would continue searching for cartridges starting at $4000.