In the Magnecor website you can note that the 10mm and 8.5m wires use the same inner core.
"Magnecor's KV85 Competition (8.5mm) and R-100 Racing (10mm) Ignition Cables feature Magnecor's exclusive 2.5mm high capacity Metallic Inductance EMI Suppressed conductor consisting of stainless steel wire precisely wound at 200 turns per inch over a ferrimagnetic core. The insulating jacket is made entirely of Magnecor's exclusive TC-1500-HS high strength aerospace grade silicone rubber, and its single layer construction will prolong the jacket's insulating ability by conducting extreme heat away from hot-spots that occur near over-the-limit heat sources.."
It's just the insulating ability that's different between the two..
" The KV85 8.5mm jacket has a service heat resistance of 600 degrees F (320 degrees C) and up to 1,000 degrees F (540 degrees C) for short burst 3 minutes. The R-100 10mm jacket has a service heat resistance of 700 degrees F (380 degrees C) and up to 1,200 degrees F (650 degrees C) for short burst 3 minutes. Even if limits are exceeded, the silicone rubber will retain its insulating ability until wires are removed from engine."
But evidently the 10mm are a fair amount of $$ more, and require some time to modify the OEM wire holders.