As to using a Saiga .308 as a DMR, the only reason I can think of not to is if it won't hang at that range. The only way to know is to try one and see.
If you check some of my old threads, you will see that I essentially did this last year. Granted, mine was using the 16" .308, however.
With quality ammunition, I was able to achieve between 1.25 to 1.5 MOA at 100 yards with the rifle. I suspect it would qualify it as "DMR" results. I could have gone cheaper, but I ended up with about $950 in the project by the time I was done.
PLEASE do not take this as a criticism of the rifle....
After all that, I decided that I wanted more of a "precision" rifle and dropped the project. I opted for another project on the LR-308/large frame AR platform where I will be expecting consistent sub-MOA performance. I was able to get into the stock rifle for what I was into the Saiga project for, but my work on the LR-308 has pushed the price tag well above the Saiga project.
All in all, I think the Saiga 308 will do exactly what it is designed to do, and it will give you much better accuracy than most AK owners will be accustomed to. However, it is simply not a platform that can be worked into anything beyond a DMR role.
This is the build that I did last year:
-- John