Hi
In most cases 8 core is slightly larger in overall diameter due (as you say)
to the extra 2 cores ,however I have recently been saddled with an 8 core
which is actually thinner than the normal 6 core due to a much thinner
conductor.If you have bought this take it back it is very easily broken.
As to the size difference in most cases clips/staples work on both 8 & 6
core and most (if not all) devices support the use of either.
You may struggle if you have double doors with individual contacts as the
slimline surface contacts don't have a lot of room.
8 core is run for many reasons in 6 wire applications inc' power doubling on
long runs due to volts drop and redundancy in case a core is snapped or goes
faulty.
Personally I only carry 8 core .(have to much other junk in the van
already).
HTH
CJ