I didn't say it isn't doable. I say it didn't really fit gameplay-wise, and "lore-wise". To begin with genemoding really imply that you're supposed tweak species differently it different places. But with PF you have to do it at once (and if you mess it up - like with colony ships, you're in trouble). Not only it severely reduce the benefits of this method, it cost a huge amount of Science to do so. Not to mention that Bio-paths techs can cause a havoc in xenophobe empire.
For a synth path - it does feel like something PF would do, but policies related to synth and their rights (or, i must say, an inability to differentiate between homemade and xeno synths) is a major roadblock here.