We have tried a lot of waterproof + breathable options and usually end up soaked with sweat. Personally, I would pack a primaloft jacket and then get a separate packable shell to pull on over the primaloft if it starts to rain. If you get a slim primaloft (but with movement for the warms to swing well while running, getting over Spartan obstacles) it will fit well under the shell.
I trail run and have done obstacles races but I have not done a Spartan and have not had to dress warmly to do a long race so keep that in mind. That said, knowing what I know, and if rain and cold were going to be an issue, I would probably do a combo like this:
(The cool green color is over 50% off right now. Actually, I'm going to buy one. That's a good deal, haha.)
With this: (also on sale at the moment)
We have jackets for all kinds of sports and weather conditions, but I picked out the mountain hardwear and rab based on price and experience with the brands.