The reasoning behind going overbore is a subject that has always interested me. I used to own a 6mm-06 AI which was like a laser. It was before the days of monolithic bullets which I suspect would have suited it very well, but chucking a 90 grain bullet that was doing over 3600 fpsat a deer was spectacular, and accurate for about 800 rounds. It nuked the barrel, which is why I started to get interested in the subject.
The 6.5 calibre is at the bottom end of the scale for a Red deer IMHO. A 140 grain bullet is about ideal for a red, providing it's of the right construction. A 140 in a 6.5mm is very high BC, dodges the wind exceptionally well and carries it's energy efficiently. The problem is it's velocity in the x55, CM or Lapua. They push it at 2600 to 2700 fps which by the time you get out to 250m is down to 2200 fps and you need a fully fragmenting bullet like an SST to kill cleanly. Come across a deer at 50m and you make quite a bit of mess.
So we try to rev that 140 gr bullet up to 3000 fps. You get into 6.5-284 territory and if you want 3100 fps you can build a 6.5 SAUM. Nosler even launched the 26 a couple of years ago which get's you to 3200 fps. It's pretty widely accepted that barrel life on the 6.5-284 is 1000 to 1500 if you treat it well. The SAUM will be under 1000 and the 26 Nosler I would be surprised if it got to 800.
Move up to 7mm and your 140 grain bullet has more frontal area so is more effective and a humble 7mm08 will launch it at 2800 fps. Back to a barrel life of 2500 to 3000 rounds. Want to go faster and a bit sexier? A 280AI will launch a 140 at 3200 fps but your barrel life will go down to 1000 to 1500 rounds, similar to the 6.5-284, but it's faster, flatter, has more frontal area and will kill further out.
A similar argument when you go up to larger species like Moose. You want a 180 grain bullet or bigger, which you can do with the 7mm but velocity is down. Move up to 30 cal and you can push a 180 at 2800 fps with an '06. If you want to push it at 3000 fps get a Win Mag and the cycle starts again.
If you take case capacity and divide it by bore area you get a factor. A number less than 1000 gives you good barrel life, over that and you are starting to shorten that life. 6.5 Lapua, 7mm08 and 30-06 are all around 850. Choose the right bullet weight and velocity for your quarry and then match it to a capacity/bore area ratio that gives you a reasonable life expectancy.
Monolithic bullets add an extra dimension. The ability to drop bullet weight because they don't fragment is redefining our chambering choice. Move from a 140 gr bullet at 2700 fps in a 6.5 to a 100 grain at 3100 fps and it will drop bigger beasts providing it's range isn't over extended. Unfortunately BC isn't flash yet so wind drift and energy loss are not as good which limits range.
Don't get me wrong, I love the 6.5 and have two of them, one for long range and one for Fallow and smaller, but I switch to 7mm for the Sika and Reds as I just find it's more flexible in terms of range and it does the job better for longer barrel life.
