Compressed cores don't reduce memory usage, and they cost some startup time, but they can significantly reduce the on-disk footprint of cores or executables with embedded cores. If that's a tradeoff you want to make, it's now pretty easily available.
To enable it, you have to build SBCL with the :sb-core-compress feature added. My customize-target-features.lisp looks like this:
(lambda (list) (list* :sb-core-compression list))
After that, you can pass a :compression option to sb-ext:save-lisp-and-die to produce a compressed core.
A quick test on my Linux/AMD64 system showed a 43MB vanilla core was reduced to about 9MB.
I plan to add support for compressed cores to buildapp very soon.