I'm in favor of enabling module compression in master & stable branches.

=============== init/Kconfig ===============
config MODULE_COMPRESS
        bool "Compress modules on installation"
        depends on MODULES
        help

          Compresses kernel modules when 'make modules_install' is run; gzip or
          xz depending on "Compression algorithm" below.

          module-init-tools MAY support gzip, and kmod MAY support gzip and xz.

          Out-of-tree kernel modules installed using Kbuild will also be
          compressed upon installation.

          Note: for modules inside an initrd or initramfs, it's more efficient
          to compress the whole initrd or initramfs instead.

          Note: This is fully compatible with signed modules.

          If in doubt, say N.
=============== EOF ===============

In particular I would go for config MODULE_COMPRESS_XZ.

It saves a lot of space:
- Size        : 364828674
+ Size        : 143548638

Surprisingly, it makes the system startup faster (systemd-analyze output of 2x3
boots):

compressed
==========
Startup finished in 2.059s (kernel) + 9.492s (initrd) + 7.496s (userspace) =
19.049s
multi-user.target reached after 7.482s in userspace

Startup finished in 2.117s (kernel) + 9.977s (initrd) + 7.483s (userspace) =
19.578s
multi-user.target reached after 7.469s in userspace

Startup finished in 2.113s (kernel) + 10.356s (initrd) + 7.733s (userspace) =
20.203s
multi-user.target reached after 7.719s in userspace

non-compressed
==============
Startup finished in 3.848s (kernel) + 9.932s (initrd) + 8.446s (userspace) =
22.227s
multi-user.target reached after 8.422s in userspace

Startup finished in 3.910s (kernel) + 10.519s (initrd) + 8.155s (userspace) =
22.584s
multi-user.target reached after 8.130s in userspace

Startup finished in 3.901s (kernel) + 8.572s (initrd) + 8.801s (userspace) =
21.275s
multi-user.target reached after 8.766s in userspace

============================================

Note that the modules are stored untouched (i.e. compressed individually) in
initrd by dracut now. This should be perhaps changed:
Size of initrd with *.ko.xz in initrd: 10758844
Size of initrd with *.ko in initrd:    10611448

*.ko in initrd and *.ko.xz on disk
==================================
Startup finished in 2.056s (kernel) + 9.665s (initrd) + 7.322s (userspace) =
19.044s
multi-user.target reached after 7.298s in userspace

Startup finished in 2.037s (kernel) + 8.131s (initrd) + 7.307s (userspace) =
17.476s
multi-user.target reached after 7.294s in userspace

Startup finished in 2.051s (kernel) + 8.624s (initrd) + 7.408s (userspace) =
18.084s
multi-user.target reached after 7.395s in userspace

============================================
This is what I did:
--- /usr/lib/dracut/dracut-init.sh.old  2019-05-22 07:52:05.296000000 +0200
+++ /usr/lib/dracut/dracut-init.sh      2019-05-22 07:56:21.600000000 +0200
@@ -903,6 +903,10 @@

     inst_simple "$1" "/lib/modules/$kernel/${1##*/lib/modules/$kernel/}"
     ret=$?
+    if [ "${1##*.ko}" = ".xz" ]; then
+           dinfo "unxz
${initdir}/lib/modules/$kernel/${1##*/lib/modules/$kernel/}"
+           unxz -f
"${initdir}/lib/modules/$kernel/${1##*/lib/modules/$kernel/}"
+    fi
     [[ $DRACUT_KERNEL_LAZY_HASHDIR ]] && \
         [[ -d "$DRACUT_KERNEL_LAZY_HASHDIR" ]] && \
         echo $ret > "$DRACUT_KERNEL_LAZY_HASHDIR/${1##*/}"



FWIW Fedora already compresses modules, but they do it in their .spec file, not
by the kernel config option:
=============== kernel.spec ===============
%ifarch %{ix86} x86_64
%global signkernel 1
%global signmodules 1
%global zipmodules 1
%else
%global signkernel 0
%global signmodules 1
%global zipmodules 1
%endif
...
  if [ "%{zipmodules}" -eq "1" ]; then \
    find $RPM_BUILD_ROOT/lib/modules/ -type f -name '*.ko' | xargs xz; \
  fi \
=============== EOF ===============

I am not sure why as Makefiles seem to do the right job:
=============== Makefile ===============
# CONFIG_MODULE_COMPRESS, if defined, will cause module to be compressed
# after they are installed in agreement with CONFIG_MODULE_COMPRESS_GZIP
# or CONFIG_MODULE_COMPRESS_XZ.

mod_compress_cmd = true
ifdef CONFIG_MODULE_COMPRESS
  ifdef CONFIG_MODULE_COMPRESS_GZIP
    mod_compress_cmd = gzip -n -f
  endif # CONFIG_MODULE_COMPRESS_GZIP
  ifdef CONFIG_MODULE_COMPRESS_XZ
    mod_compress_cmd = xz -f
  endif # CONFIG_MODULE_COMPRESS_XZ
endif # CONFIG_MODULE_COMPRESS
export mod_compress_cmd

=============== scripts/Makefile.modinst ===============
quiet_cmd_modules_install = INSTALL $@
      cmd_modules_install = \
    mkdir -p $(2) ; \
    cp $@ $(2) ; \
    $(mod_strip_cmd) $(2)/$(notdir $@) ; \
    $(mod_sign_cmd) $(2)/$(notdir $@) $(patsubst %,|| true,$(KBUILD_EXTMOD)) ;
\
    $(mod_compress_cmd) $(2)/$(notdir $@)