What is the relative power of GWAS studies in different species?
Part of the increase in power to detect a genetic association via GWAS-genotyping comes from long haplotypes. Many dog breeds went through selective breeding bottlenecks 100-200 years ago. Many lab model organisms, such as flies, worms and plants, have recombinant inbred lines that aid in GWAS discovery as well. The price one pays for this increased power, say for canine GWAS, is not being able to pinpoint the casual variant. The association SNP(s) found may be far, far away from the gene responsible for the phenotype, because of that long LD.

Another aspect not to neglect is the phenotype. In humans, there may be a 3-fold range in phenotype p, while in dog that value is just 35% (1.35x). That may mean that the phenotype is difficult to measure (accurately) and to assign to a given allele with statistical certainty.

A third aspect to consider is the effect size of the association, say on a per (risk) allele basis.

Thus, while I am not aware of any papers specifically taking such elements as haplotype length, phenotype range and effect size into an analysis of GWAS power across species, I feel that one versed in genetics of the organism of choice can readily accomplish this for the specific trait(s) of interest.