Don't rely on export/import for a production system as a method of backup. Period.

The performance degradation of a hot-backup depends on the amount of activity that is going on in the database. If you do your backup when the activity is low, you will see a small performance difference (<10%).

Running in archivelog mode will barely impact performance if you have fast enough devices. If your archives are kept on the same disk as your TEMP/RBS/Redo/DATA, then it will impact performance significantly.

I am assuming you have a reason for only backing up one tablespace... I couldn't justify it to myself, but it sounds like you thought about this...