Will one cable upgrade make a noticeable difference?

I have transparent cables and also have the power isolator. I would call customer service in maine for insight.  I found them especially helpful. Also got power cables from them as well and interconnects.

Overall it makes me feel more comfortable with what I'm listening to and reduces the need to be critical.  If you have a great system it will help things a lot better. But it does bring out recording quality differences which is annoying in its own right. 

And the farmer hauled another load away !! 

Question: Would replacing the SR with either a Transparent G5 Reference or G6 Ultra make a noticeable difference in the sound?

Likely Yes, no guarantees.

Or other suggestions? 

You have a very well balanced system.  Upgrading PCs would likely be positively noticeable, but low of diminishing returns it may not be worth it.  Maybe try/borrow a higher quality PC to try on each component.  Otherwise, the change would have to be your choice - changing the speakers or electronics.  

Another option is to add vinyl.  I have both.