Its very likely that Precomp will help you. Use it like this:
(If it takes too long, try it without the intense switch)
precomp -c- -intense filename.iso
The output will be a file named filename.pcf. Precomp will display some statistics - if it finds streams and the file gets larger, you can compress this PCF file instead of the original ISO file to get better results. Restore the original file using
Additionally, have a look at SREP - using a "tool chain" like Precomp->SREP->7-Zip usually gives good results with fast decompression.
precomp -r filename.pcf