Will running graphics/hardware intesive programs lower the "life span" of an iMac?
Mac hardware is designed to handle its maximum load without being compromised, so theoretically the answer is no. 

In practice, as everything electronics or mechanical, if you push towards its limits you machine, every parts of its is subjected to major load and stress.
Wide variation in temperature can provide degradation to silicon properties and may cause failure or T-junction breakages if the hardware temperature watcher doesn't switch off CPU in time. The same is true for GPU. 
From the point of view of HDD and SSD, they have a limited number of writes, and they are very sensible to temperature variation. In fact if you expose for long periods an SSD to temperature higher than 50 degrees, this can half is life. 
Moreover, the more energy you hardware require the more fast you battery will drain, and the more often you have to reload the battery. Here you are contributing to increment the battery load cycles. Batteries have a limited lifetime before they lose their load capacity (it's around 900 cycles).