Chafe is ONLY caused by contact with other stuff between the 2 attachment points, and then when it does, chafe depends to some degree on the type of rope is used. I would say the construction of a rope has very little to do with that.
I have to add that there are some double braid ropes out these where the outer 'mantle' is for protection and the inner core provides the strength.
Back to your 8 plait: Go for it!, it remains subtle, although it is harder to splice than common 3 strand.
When you want to splice your 8-plait, here a good link how to do that by Sampson http://www.samsonrope.com/Documents/Splice%20Instructions/8Strand_C1_Eye%20Splice_JUL2012_WEB.pdf[/URL]
Regarding strength: indeed volume of rope material per linear ft or mt reflects the strength, but in my experience the difference is little (ie less than 10%) when using quality ropes. Just check some stats on-line and see if the rope you have is strong enough. Note that nylon can absorb big shocks far better than any other kind of rope. If the rope is too heavy/thick/strong it will NOT stretch much anymore and it becomes also more difficult to handle and stow.
In my case (10-11 ton 34 ft ferro boat) my anchor line is 16 mm nylon 3 strand, my docking lines are 14 mm double braid, outer braid polyester, inner that of nylon. But when I can off-load my 3 strand, I like to replace that with Nylon 16mm 8 plait, that will remain softer and easier to stow than the 3 strand.