At the bottom are some interesting links to info about the 6.8 SPC and the different chambers, the original, specII & DMR.
If you cut anything other than the leade of the chamber I think you would have to head space the barrel to the bolt again. The DMR chamber looks to be slightly different in specs than the Ruger chamber (original) at a critical point and may not match up with the new reamer, so you would have to cut back the shoulder of the barrel & re-time the gas hole to the receiver, lot's of work.
I have been going around with the 6.8 over the mini 30 for awhile now trying to decide if it is worth the leap to go to the 6.8, even if you have to modify the chamber a bit to get the velocity the 6.8 is capable of. The 6.8 out of a mini even according to Rugers catalog are far below what the round was first claimed to be. Ruger says velocity of the three different Remington 115 grain loads for in the mini 6.8 average out to be 2450fps that is a resulting energy of 1533 ft lbs. That is far below the Remington claims in its ballistics figures of 2650 fps. Why the difference I don't know, but these are Rugers own figures from pg 60 & 61 of their 2008 catalog. Seeing that the AR crowd is getting their max velocity out of 18" barrels seems to let the barrel length out of the equation.
2350 fps is easily attainable out of a mini 30 with a 125grain bullet for the exact same energy 1533 ft lbs. So the only advantage of the 6.8 out of a stock mini that I see is trajectory and better retained energy at longer ranges.
But if you modified the chamber of the 6.8 mini to take the higher pressure that the round was designed for then it would be worth it. 2650 fps or 2750 fps would make the round much more viable than the 7.62x39, making it harder hitting at close ranges and flatter with better energy retention at longer ranges.
If I were going to do it I would probably put witness marks on the barrel/receiver connection, pull the barrel, have the leade increased to the specII chamber specs with a .270 caliber throater (about 50 thousandths more) (and have the barrel cyroed as long as it was out) reinstall the barrel to the witness marks and see what happens. That way I'm not messing with cutting the chamber and messing up the head space. The extra leade would allow it to handle the higher powered combat loads from SSA, according to Art Kalwas, the owner of Silver State Armory. I wouldn't mess with trying to cut a new chamber as much more work would be required to reinstall the barrel. The other thing the mini has working against it is the twist and number of grooves in the barrel, unless you went with the type of barrel that the AR people are using, with a slower twist and less lands & grooves to reduce friction to increase velocity without building up pressure you will never be able to SAFELY get the kind of velocities the AR people are getting from the 6.8 which is right at 2800 fps & 2000 ft lbs of energy with a 115 grain bullet.Still if you can get 1750 ft lbs of energy with a slick 115 grain bullet at 2650 fps I think it would be worth it.
Anyway these links have a lot to offer if you take the time to read them.
The 6.8 SPC (6.8 x 43 mm) Cartridge
be sure to click on the link in the article for the chamber drawings, remember you are seeing the chamber not the cartridge.
Today's State of the Art in 6.8 X 43 mm SPC
good read lays out the 6.8 story.
Here is a link to the Reload Bench
which has drawings of the 6.8 SPC CARTRIDGE, as well as some others. scroll down to the metric section and click on 6.8SPC.