http://www.carmax.com/enus/maxcare/default.html
Claims that all parts are covered except those listed. Then goes on to give an elaborate list of what is covered. Which is it? IMS not listed.
Claims non-covered parts covered if the result of covered part failure. No claim of covered part covered as the result of non-covered part failing (as in IMS taking out engine if IMS not covered).
At the bottom reveals Porsche and CarMax aren't behind the warranty, it is just a warranty from one of several third parties. Research those companies.
Read the contract...not the sales literature.
If it is as you hope, the price and length make it a great deal. It is those little details that always worry me and I hear from my lawyer friends about.