I'm a professional coach and I have seen beginner (27+ min 1500) to intermediate swimmers (~22-26 min 1500) improve by 10% or more and advanced swimmers (under 22 for 1500) improve by 1-2%.
Total Immersion is not the end-all-be-all of swim technique, but it is hard to argue with physics. One of my clients got a side view video ot Ian horpe at the lasy Olympics and it was amazing how closely he followed the TI principles.
Good luck,
Ken (another Hokie)
www.Fitness-Concepts.com