I assume no new chain or adjustments to the Di2 anything else either moving from the 12-28 to the 11-28?
From what I read a few times is that gearing could be too close on the 11-28?
Also I've read that the 11-28 can give faster speeds in the flats?
Do I loose much on hills with the 11-28?
Not sure what someone would mean by "too close."
The 28-tooth gear is what you will use when climbing, so you will lose nothing on the hills. The 11-tooth will be the "top speed" gear and is technically faster than the 12-tooth, but you have to be a pretty good rider to be able to spin out in the highest gear!
The chain length should be OK unless they sized it in an unusual way. Make sure to try the 11-tooth sprocket with the smaller chain ring and see if it shifts into and out of that gear without any interference.