Sorry for the delay. Initially purchased some cheap cables just to get the build completed. However I didn't like the white connectors and the colored cables, so it wasn't long before I upgraded to a set of Noctua sleeved cables. These look nicer, and been running great ever since.
Sorry for the delay, hardly come on here these days. To answer the question, the i5 is holding up very well. Definitely don't regret opting for it over an i3. To my knowledge (and I'm no expert), the turbo boost is automatic, and kicks in when needed. I've certainly never had any performance issues with this chip, and I think it would be a good upgrade on your G3258. Hope this helps!
Ha! I'm not a kiwi, I just live in NZ. Good luck with your build. This rig is still going strong, and runs like a dream.