I do not know the answer to your question, but in the case of human disease high GC content has been linked to increased susceptibility to genomic rearrangements (see Hum Mutat, 22, 229-244, 2003; Genes Chromosomes Cancer, 43, 383-389, 2005). However, very high AT contents (close to 100%) have also been associated with recurrent translocations (see the work by Kurahashi, Emanuel and colleagues).