First off, a Big Congratulations to OP. Well earned kill, excellent job.
Enrage timers add nothing to a boss fight....can you imagine actually fighting one of these 'bosses' in real life, and if you didn't defeat it in a man-made unit of time it suddenly bursts into flame, and destroys you? Its much more interesting when its, I can do this, and the boss can do that, how do I overcome it? The winner, regardless of time, will be chosen.
The only enrage I'd ever want to see on a boss, is something like as health decreases they gain strength or abilities; but then again doing that doesn't really make it an enrage, just a part of who that boss is.