you can probably get away with the 45 gn varget load with the 178's as well, at least 44 though--the above loads given are very mild. I would push the 168 to 2800 and the 178 to 2700. that can be done safely with a 22" tube and the right powder. Also 42 of benchmark seems alright with a 168smk and 175 berger. The amax should serve you well to 800yds.
42.5 varget with a 165, man that cant be goin too fast - 2500fps maybe?
i just like to get the most velocity out of my 308 i can, since you're starting with so little to begin with.