I ski K2 Apache Recons and had a hard time skiing esp. in the crud.
Is it safe to assume that fat skis handle the crud and powder much better than a 50/50 ski?
When exactly should I consider moving to a fat ski (which I would just rent as we don't have a need for them in my area except for end of season)?
What you're hearing here is correct. In any kind of soft conditions, it's likely that you'll be happier on a wider ski that keeps you more on top and steers more easily. You should demo, though. Some fat skis get knocked around badly in dense crud, while others slice it up just fine. (As a Pacific Northwest skier, I have encountered dense crud on occasion!)
That said, you may find that fat skis are difficult to rent or demo on powder days. Everyone else has the same idea, and the competition for the available skis can be fierce.
I also own a pair of Recons. I find them to be a soft, damp versatile ski, and I've always thought they handled crud just fine when on edge, but not so well when they're flat. But that's typical of "narrower" skis. And, in fact, Recons don't do much of anything well when they're flat, except pivot slips. They weigh a ton with the Marker system bindings, though, and that helps them plow through junk. Also, I've never skied more than about 36" of untracked with them. Hey, what can I say? They arc just fine even when completely buried, I got a face shot every turn, and I didn't own anything wider at the time. I do now.
It's hard to say exactly when you should consider moving to a fat ski. Since you characterize yourself as an intermediate, I suspect (but don't really know) that you might find a fat ski in 2" of fresh snow fun and somewhat easier than your Recons, since the fat ski will pivot and smear more easily, even in just 2" of new snow.
Anyway, try them. You'll like them. And if you're headed West, you may want to arrange your rentals in advance. You may not get freshies, but you can most likely find softer snow than you find in the East. That by itself will give you a chance to experience the difference a fat ski will make for you.