>b) call an external compression program (zip, arj)

Can be easily done with -V option (-V "tar uvfz 
foo.tar.gz \$0") under Linux - BUT  in this example, 
taring is really slow

>c) some other ideas

Using the uder-defined structure, like %h%p/dir%q/%n%
q.%t or %h%p/dir%t/%n%q.%t but I suspect this solution 
to be useless, as it will generate too many 
directories, too!

Large websites and filesystem limit is a problem, 
especially when getting a large number of files in a 
common directory (the worst thing is those
foo.cgi?page=1,foo.cgi?page=2,...foo.cgi?page=999999)