As you say of external, a major benefit of using external is removing 32bit limitation. srep+lzma is always better than srep+mx or soo, if you are not using old srep versions as srep 1.5. Yesterday I tested this on a 513mb sample file. "513mb" so rep can on work on it instead of srep. First I applied mx and got 454MB on that file. Then I used srep+lzma, which didn't took much time and gave me 452mb. These 513mb files got some executables, some texts and some other files and clearly fa's grouping didn't benefited me anyways. srep+lzma is not always good, if you want multimedia and other detection and use algorithms as groups, you must use masked data compression. You can find it
here