{me SUMTI mei} is awesome and it seems like we can nicely gloss it as "x1 is an amount (on an unspecified scale) represented by SUMTI. IMO, however, the x2 should simply be the type of stuff being quantified. This makes it hold more consistently with the BPFK definition of numeric mei. Indeed, rather than numerically quantifying some x2s, which is what the numeric version does, we can vaguely quantify by comparison with another object.
e.g. {.i mi pinxe lo me lo xance mei be lo djacu} {.i mi citka lo me lo palte mei be lo cakla}