Hey @Magnito I would say that it is certainly achievable. I achieved a 12% improvement in my time on a 20km loop within 4 months and I was only halfway through the All Purpose Road plan at the time. I also lost 4kg over that period by cutting down on carbs and cutting out breakfast. Clearly not doing a 170km ride the day before would help your TT time.
You should also check this post out: From the Coaches: Pacing for Performance It would indicate that your FTP is fine and timing your effort is the key to success (along with nobbling the club World Tour pro of course)