Cutting out a stem is just not that big of a task. One hacksaw, plus a 1/2" drill bit (or something like it) to get it started, and in about 10 minutes, its out.

While I am a chemical engineer, and love using various chemicals to accomplish tasks for me, sometimes, mechanical means are best.

The brute strength methods can result in damaging the steering tube, the fork, or the frame (depending what you are working on, and where you are applying the force). While I will still try Kroil briefly, I will not put much time into it (squirting Kroil on it a few times = 2 minutes of time invested, and if successful, you also do not destroy the stem or seat post).

Quote from above: "If you had done it my way you wouldn't have chopped it off."
+1 How many stuck stems or posts have you successfully gotten loose? At one point, I was 100%, I really thought I could work magic. Then I got my butt handed to me. Check with someone who has handled hundreds or even thousands of neglected bikes, like Randy Jawa and others.