It has long been recognized that there is a natural dependence between theories of knowledge representation and theories of knowledge acquisition, with the idea that the right knowledge representation enables acquisition to happen as reliably as it does. Given this, a reasonable criterion for a theory of knowledge representation is that it be useful for acquisition, particularly in nontrivial learning situations. We propose quantitative learnability metrics meant to capture how useful a representation is for acquisition. We then apply these metrics to the case study of English metrical stress, a language that is notorious for having nonproductive aspects in its grammar and so being nontrivial to learn a productive grammar for. We examine three theories of metrical stress representation, assessing their learnability via these metrics from English child-directed speech at different stages of linguistic development. We find that while all three theories are only somewhat useful at the initial stages of stress acquisition, they are far more useful at later stages and define a grammar able to capture the vast majority of English children's acquisitional intake. Interestingly, we also find that the proposed English grammars in each representational theory are not the grammars most easily learnable from English child-directed speech. Instead, minor changes to each English grammar yield an English-like grammar that is far more learnable, suggesting that these alternative English grammars should be given further theoretical, experimental, and computational consideration. We discuss implications for both theories of representation and theories of acquisition.