The problem with your argument is that most plants don't actually have larger genomes. Just some of them. So, if we found a plant with a small genome that synthesized a large range of chemicals, than we could simply assume that the plants with larger genomes aren't getting that kind of benefit. And that is indeed the case. Nobody has found anything phenotypic in these large-genome plants/fish that shows them to be more complex or anything.

My PhD is in Biophysics, my BA in Biochemistry/Molecular Biology, although I don't work in botanical bioinformatics, my education provides me with the tools to make rational decisions based on data.