You need to take into account that using proportional representation, people would vote differently than they do now. People would be more willing to vote third party, since they would have a higher chance of getting someone in that party into office.
^^ Also this all depends on the method of PR used, whether it is a list system (if so whether it is open or closed) or whether they use Single Transferable Vote etc.
Note also that there is often a threshold in the system required to be achieved before you can get someone elected set at anywhere between 5 and 15 percent normally.
