Like Fly S, I am only commenting from my own experiences.
I think 15 hours in the Seneca is ample. Myself and two of my friends both completed the IR course with the 15 hour minimum, and we weren't topguns, we just worked hard. By the end of the course you will feel comforable with the aeroplane. As Fly S said, continuity is important for you to keep moving forward.
One of the only advantages I had is I did do my multi engine rating on the Seneca 3.
Good Luck with the course.