I suggest redirecting the results into a cachefile, because the extraction is pretty slow.

srcsysmod_kmod() {
  tab="$(printf "\t")"
  (
    cd /usr/src/sys/modules
    find . -type f -name 'Makefile' | sed -e 's#^\./##' | xargs dirname | sort | while read dir
    do
      # Slower but more precise
      #kmod="$(make -C "./${dir}" -V KMOD)"
      # Faster, but might be incorrect if the makefile does voodoo
      makefile="${dir}/Makefile"
      kmod="$(sed -e "/^KMOD/"\!"d; s/^KMOD[ ${tab}]*=[ ${tab}]*//" "${makefile}")"
      # Format and display the results
      [ "${kmod}" ] && echo "${dir}${tab}${kmod}"
    done
  )
}

Here are some more methods that I'm building to figure out how the 'device foo' lines in the kernel config relate to what modules no longer needs built

# Format:
#       /*
#        * include "filename"
#        * filename    [ standard | mandatory | optional ]
#        *      [ dev* [ | dev* ... ] | profiling-routine ] [ no-obj ]
#        *      [ compile-with "compile rule" [no-implicit-rule] ]
#        *      [ dependency "dependency-list"] [ before-depend ]
#        *      [ clean "file-list"] [ warning "text warning" ]
#        */

depfile() {
  [ ${#} -gt 0 ] || set - /usr/src/sys/conf/files /usr/src/sys/conf/files.$(uname -m)
  expand "${@}" | sed -ne '
    s/^ *#.*$//g
    s/ *$//g
    /[^\\]$/ { H; s/.*//g; x; s/^\n//; s/\n/ /g; s/  */ /g; p; }
    /\\$/ { s/\\$//; H; }
  '
}

depfile_devices() {
  depfile | sed -e '
    / standard$/d
    / standard /d
    s/ no-obj .*$//
    s/ compile-with .*$//
    s/ dependency .*$//
    s/ before-depend .*$//
    s/ clean .*$//
    s/ warning .*$//
    s/ *$//
  '
}

devices() {
  depfile_devices | sed -e '
    s/^.* mandatory //
    s/^.* optional //
    s/|/ /g
  ' | tr ' ' '\n' | sort -u | grep -v '^$'
}

device_files() {
  device="${1}"
  depfile_devices | sed -e '
    s/ mandatory / optional /g
    / optional '${device}'$/s/ optional .*$//
    / optional '${device}' /s/ optional .*$//
    / optional .* '${device}' /s/ optional .*$//
    / optional .* '${device}'$/s/ optional .*$//
    / optional /d
  '
}

srcsysmod_kmod_files() {
  tab="$(printf "\t")"
  (
    cd /usr/src/sys/modules
    find . -type f -name 'Makefile' | sed -e 's#^\./##' | xargs dirname | sort | while read dir
    do
      kmod="$(make -C "./${dir}" -V KMOD)"
      srcs="$(make -C "./${dir}" -V SRCS | sed -e "s/${tab}/ /g" -e 's/  */ /g')"
      # Format and display the results
      [ "${kmod}" ] && echo "${dir}${tab}${kmod}${tab}${srcs}"
    done
  )
}