One more note, if you change to a triple crankset, you will need a new front derailleur to make it work right. I don't have experience with ST-6500, but other similar Ultegra models, like the 6600, had a double and a triple version and I would be surprised if the existing double left shifter would work.

A triple crankset will also require a different bottom bracket. And it will require a long cage rear derailleur.

So here's your shopping list to switch to a triple:

Triple Crankset ($100)
Longer spindle Bottom bracket ($30)
triple front derailleur ($25)
left shifter ($85)
handlebar tape ($15)
long cage rear derailleur ($32)

And after all that your low gear will 28/23 (1.28) which is actually a taller, harder gear than the 39x34 (1.15) you could get for under $100.

Last point, the difference between a 175 crankset and a 172.5 is 2.5 mm or about the thickness of a sock.