I got a strange thing here, using SQL Plus as SysDBA is some what different than IMP.
To specify the connection string; you need the following format: username/password@instance as SYSDBA.
For SQL Plus:
This should be wrapped by double quotes "username/password@instance as SYSDBA"
This should be wrapped by single quotes 'username/password@instanceas SYSDBA' ; double quotes will not work.