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I was wondering if its worth using just to keep the temps down while its not being used?
Yes, and lower power consumption.
Is there a noticeable sluggishness going from down clocked to max speed?
No.
That sounds a strange issue - on the game that is lagging try setting a core to it using affinity:
http://www.tomshardware.co.uk/forum/244402-10-assign-programs-single-core