The Plumbing Efficiency Research Coalition (PERC) recently released a study on the effects of low-flow toilets on drainage systems. The main takeaways from this study are that 1.6 and 1.28 gpf toilets have no significant detrimental effect on drain line flow, but 0.8 gpf toilets were shown to have a high percentage of blockages. Interestingly, flush velocity has little impact on drainage, but toilet paper selection has a large impact. They recommend using cheap single-ply paper for the best performance.
Check out the full report here.