1:8 will handle any common .223 bullet that when loaded into a cartridge will fit into a standard magazine. This equates to 77gr SMK and 77gr Nosler, for the most part, as well as anything lighter with the possible exception of the Hornady 75gr A-Max Match (not the 75gr BTHP match).
Really, 1:8 is a very practical twist rate, it's a shame that it isn't more common. It is fairly easy to find in heavy stainless steel "match" type barrels from a number of manufacturers. Look at Bravo Company and CMMG for some starting options.
1:7 was the military choice to get M856 tracer to stabilize under the coldest arctic conditions. It does that, but is excessively fast for any other use including what 99.999% of us civilians are likely to want to do. It will also stabilize the ultraheavy 80gr and heavier match bullets, which won't fit into a standard magazine when loaded in a cartridge.