Strictly speaking the domain of these comparative Postmodifiers is not the Head of the group but an item within the Premodifier: as, more, less, too (the exception is -er comparatives like faster). This could be shown as in Figure 6-25 (a); cf. the nominal group, where given a better man than I am we could show than I am as dependent on better rather than on man.* (* Cf. the brightest star in the sky, where in the sky would modify brightest.)
But this is not really necessary: structure is not the appropriate concept for interpreting semantic domain, and the locus of comparison may in any case be part of the Head (the -er in faster, readilier) or even part of the Postmodifier (the exceptional form enough, which follows the Head). It seems unnecessary to represent pairs such as too fast (for me) to follow, slowly enough (for me) to follow, or as fast as I could count, faster than I could count, as having different structures. They can be analysed as in Figure 6-25 (b).