My HS likes the 154 gr Lapua Scenars but I have to push them fast at long distances. This is harder on the brass and the barrel. The 175's buck the wind better but the 154's have a better BC. 6 of 1, half dozen the other I guess.
The 154's like to sit closer to the lands in my gun, closer than I have to run the 175's. The longer load on the 154's just squeek into my mags and don't feed the best. If you sit the 154 beside the 175 SMK the Lapua is actually longer.
I don't know about what kind of luck others have had but my rifle sucked big with the 168 SMK, and the greater distance the shot, the worse the groups were. Past 400m they were no good. The 154's and 175's solved that problem. The 175's being less fussy to load.
As Jamie pointed out, you can use a slower burning powder with the longer barrel (Varget or RL15, Varget being less tempurature sensative). He will have better luck with IMR as its burn rate is faster, generates the pressure he needs in the shorter barrel before the bullets out the business end.
Good luck, try some different stuff and see what works.