Time to replace my upper and lower motor mounts and I am thinking about adding powerflex inserts to both. Do they increase service life of the mounts or not?

 

Seeing as how your original ones lasted ~20 years, probably not. That's a really good run for a piece of rubber in harsh conditions

 

I had installed the lower powerflex bushing into the original lower mount (PFF5-120) while not the insert kind it failed within 100km. Spoke with Powerflex who had agreed to replace it, but we opted to upgrade to the entire (PFPFF5-125P) and i would pay the difference in cost.

I would not hesitate to install anything powerflex, they will stand behind their product. You will however pickup some vibration from inserts, but they will ultimately help make the rubber last. Like the plagued r56 upper mount, that BMW decided to put into all sorts of vehicles... Even with poly inserts it wont last, but without them your doing bi monthly mount changes if you track your car in anyway...

Go for it!