My main question was - will Heroic and Mythic raid gear be able to upgrade past Warforged?

Since "past Warforged" just means "by more than +10 item levels over base," the answer to that is "yes."

Let me use some entirely made-up numbers as an example:

Let's assume that the chance to upgrade is 50% per +5 step to simplify the math. Clearly that is NOT the actual chance, as anyone who's been playing at max level on Beta can attest. But let's pretend that whenever an item drops, we flip a coin and bump the item level by +5 every time it comes up Heads. And we continue until we either flip a Tails OR the item reaches Item Level 895.

Let's consider three Emerald Nightmare drops: one from Raid Finder (835+), one from Heroic (865+), and one from Mythic (880+).

The Mythic Nightmare raid drop is guaranteed to be at least 880. We flip our imaginary coin (again, these are NOT the actual odds...). 50% of the time it'll come up Heads (woo, 885!), so we flip again. Again, half of the time we get this far (25% chance total, now) it'll come up Heads again (890!). And so we flip yet again, and yet again half the time we make it this far, we'll get yet another Heads (12.5% total). That bumps the item to 895, which is the cap, so we stop flipping coins and the item ends up at +15 over the base 880, which makes it Titanforged.

Using these hypothetical, made-up numbers, if you looted the same item from a Mythic Nightmare boss 8 times, you'd expect on average to see four 880 versions, 2 885 versions, 1 890 version, and 1 895 version.

Now let's look at the Heroic base item. That starts at 865, so you'd need to upgrade it six times (i.e. flip six Heads in a row, using our analogy) for it end up at 895. I'm not going to step through the probability tree again, but the 1/8 chance above now becomes a 1/64 chance. You'd need to loot 64 Heroic Nightmare raid items on average to see a single 895. The Titanforged chance (again, not the actual odds!!) would still be 1 in 8 (12.5%), since that just means 3 successful consecutive coinflips, but that would only bring the item to 880. You need to get much luckier to get all the way to 895.

Finally, our Raid Finder item. With a base Item Level of 835, it needs to upgrade 12 times to make it all the way to 895. Long story short, that's a 1 in 4096 chance of being 895. Let's say you get 2-3 items a week from fully clearing Raid Finder. You'd need to run Raid Finder every week for over 21 years in order to have a >50% chance of seeing a 895 item out of Emerald Nightmare LFR. By that time, we'll all be in item level 4000 gear as we play the latest Corgis Unleashed expansion on our VR decks (unlike the rest of this post, this part is entirely factual). But hey, you'll probably have that 895 Raid Finder item. Or you still might not. Still feel mandatory?

As this example hopefully illustrates, you're best off spending your time on the most challenging/rewarding content that you can tackle. This isn't destroying the foundations of WoW itemization and replacing it with massive randomness - by and large, players will progress through content of increasing difficulty and/or organization requirement, earning more and more powerful items as they go.

Titanforged In Raids as well?