It depends on how the program is coded. If it is coded to handle multiple threads then I would say it would help, if it is a single-threaded application then it wouldn't make any difference.
One way to test would be to try uncompressing and see how quickly it does it with your current PC (assuming it is a C2Q, for example) and then disable two of the cores and run it again, and see if you see a big difference. It may not be a perfect way to do it, but it may give some insight if you don't know if the app is multithreaded.