An addendum for comment#49 above at
https://bugzilla.novell.com/show_bug.cgi?id=735404#c49

Regarding
"weird filtering e.g. of PostScript input via pstotiff and imagetopdf":

With HPLIP's default entries in its /etc/cups/pstotiff.convs
------------------------------------------------------------------------------
application/postscript  image/tiff      80       pstotiff
application/pdf image/tiff      80       pstotiff
application/vnd.cups-pdf image/tiff 80 pstotiff
application/vnd.cups-postscript image/tiff 80 pstotiff
------------------------------------------------------------------------------
one gets this weird filtering of PostScript input:
------------------------------------------------------------------------------
# echo hello1 | a2ps -1 -o- | lp -d HP_LaserJet_1020
[stdin (plain): 1 page on 1 sheet]
[Total: 1 page on 1 sheet] sent to the standard output
request id is HP_LaserJet_1020-58 (0 file(s))

# grep -o 'Job 58.*PID.*' /var/log/cups/error_log
Job 58] Started filter /usr/lib/cups/filter/pstotiff (PID 3949)
Job 58] Started filter /usr/lib/cups/filter/imagetopdf (PID 3950)
Job 58] Started filter /usr/lib/cups/filter/foomatic-rip (PID 3951)
Job 58] Started backend /usr/lib/cups/backend/hp (PID 3952)
Job 58] PID 3949 (/usr/lib/cups/filter/pstotiff) exited with no errors.
Job 58] PID 3950 (/usr/lib/cups/filter/imagetopdf) exited with no errors.
Job 58] PID 3951 (/usr/lib/cups/filter/foomatic-rip) exited with no errors.
Job 58] PID 3952 (/usr/lib/cups/backend/hp) exited with no errors.
------------------------------------------------------------------------------

When all entries in its /etc/cups/pstotiff.convs are disabled
------------------------------------------------------------------------------
#application/postscript  image/tiff      80       pstotiff
#application/pdf image/tiff      80       pstotiff
#application/vnd.cups-pdf image/tiff 80 pstotiff
#application/vnd.cups-postscript image/tiff 80 pstotiff
------------------------------------------------------------------------------
plus restart of the cupsd so that it re-reads its *.convs files
one gets back the normal CUPS upstream filtering:
------------------------------------------------------------------------------
# echo hello2 | a2ps -1 -o- | lp -d HP_LaserJet_1020
[stdin (plain): 1 page on 1 sheet]
[Total: 1 page on 1 sheet] sent to the standard output
request id is HP_LaserJet_1020-59 (0 file(s))

# grep -o 'Job 59.*PID.*' /var/log/cups/error_log
Job 59] Started filter /usr/lib/cups/filter/pstopdf (PID 3997)
Job 59] Started filter /usr/lib/cups/filter/pdftopdf (PID 3998)
Job 59] Started filter /usr/lib/cups/filter/foomatic-rip (PID 3999)
Job 59] Started backend /usr/lib/cups/backend/hp (PID 4000)
Job 59] PID 3997 (/usr/lib/cups/filter/pstopdf) exited with no errors.
Job 59] PID 3998 (/usr/lib/cups/filter/pdftopdf) exited with no errors.
Job 59] PID 3999 (/usr/lib/cups/filter/foomatic-rip) exited with no errors.
------------------------------------------------------------------------------

The reason why HPLIP's default entries in its /etc/cups/pstotiff.convs
mess up the normal CUPS upstream filtering is:

When HPLIP adds CUPS filtering rules via a *.convs file
for standard MIME types like application/postscript and
application/vnd.cups-postscript then those rules become part
of the standard CUPS filtering which means those rules
affect all print jobs with that standard MIME types.
In short:
What CUPS filtering happens depends on the MIME types so that
for same MIME types same CUPS filtering happens and
when HPLIP needs different CUPS filtering rules
then HPLIP must use different MIME types.