Ported to a PC, and working on a PC for a demo I wouldn't call identical things. Also worth noting that Oxide took one guy two months to get Mantle working(though not completely optimised) for their engine, with other dev's claiming about the same.
In general people are making a bit of a fuss over API's as if they are the hardest thing in the world to write for or adding support for a new API is massively costly, it's not. Mantle was pretty cheap to add support for, DX12 will be. Where the cost/issues arise is making new features to take advantage of the extra power.
The real question is why didn't MS do this years ago.......... and I think most of us know the answer to that.
Still quotes about DX12 making Xbox one way faster.... too many quotes really. Considering the existing Xbox api should be lower level and faster than DX12 could ever really be, and considering it has to factor in low bandwidth and edram to get the best performance I simply can't see how on earth a higher level API written with more abstraction will be faster.
Either MS are really desperate to con people into thinking the Xbox will magically unlock loads more power in 18 months, or they've absolutely well and truly screwed up their to the metal API on the xbox. The former seems like 90% more likely, I pretty much discounted the latter as screwing up that badly would be so bad I think they would have delayed the console till they fixed it.