You cant put a bigger rear shock on without doing mods to the frame. The bike is designed to work around that particular travel. If you want a longer travel bike look for the supreme dh not the 6/mini. Better off having a full blown dh bike if thats what you are after.
The commencal design is very good at making the bike feel like it has more travel than it does while maintain a very good pedal platform as if it has less travel, It works great. Only neg is you get brake jack but thats easy to work with or around once you get used to it.
If you have the cash a van r in the rear or rc4 will do wonders for the bike. After that it is just a mater of getting your bar width, stem length/height and of course suspension set up. If you get the headtube sleeve with this bike throw on the -1 to slacken your HA by 1 degree.
The bike pictured has got a few spacers under the stem and the bars look like they have quite a bit of rise on them, Personally Id get the spacers out and potentially throw on some low rise bars if needed. Your set up is the most crucial part of getting the bike to feel good, 6inch is plenty to thrash a dh trail on.