Ok, I am struggling. This is what I have and then this is what I want to do. Can I do this?? If Ican not, what are my options as I dont want to buy a new OSPW!!
Current 2x 11sp mech:
11/28 rear cassette (Dura-ace)
2 x 52/34 chain rings (ultegra)
Ultegra R8000 SS Rear derailer & OSPW
Wish to do 1 x 11sp Di2:
11/28 rear Cassette (Dura-ace)
1 x 54 chain ring
Ultegra R8050 SS Rear derailer & OSPW
Of course cahnging it all to a Di2 system but this is about the size of cassette and chain ring
What should prevent you from doing this exactly? I am not sure I understand your question? Are you asking whether you have the right gearing (answer: depends on the terrain and how strong you are)?
Also, if you change your rear derailleur anyway, why not go 12-speed? You could get a wider-range cassette that way.
I dont know if it would work with the differences. This isnt an area i understand with cassette and front ring differences. Angle of chain etc…tried to get my head around the derailers capacity as well but it all goes over my head. Basically I need someone to say it would work. Rather me tryingvand then finding out it doesn’t…
I guess inam asking if there is any problems doing this. Legs definitely can do it. Nect year there are no hills on any of the races i have planned and i literally have half of the year to build strength.
What is stopping me is minimising the costs. I would loose the useage of parts i alresdy have. Like my expensive OSPW and Duraace cassette. That would be an extra $1200…
Why not just get a 1x chainring and skip the Di2 for now?
The chainline issues are resolved with a 1x chainring.
If you’re set on the Di2 setup, I’d just sell off whatever you won’t need moving forward. 
Let me second @eddiegrinwald’s suggestion: why do you want to spend €€€€ to make the switch to Di2 if you stick to 11 speeds. You can test the ideas with your mechanical setup by purchasing a 1x-specific chain ring for your cranks and taking off your front derailleur. It will just work.
Garbaruk makes a variety of 1x chain rings (round and oval), if you run Ultegra 8000-series cranks (which seems to be the case if I understand your post correctly), then these are the right ones.
If you upgrade to Di2, I’d go to 12 speeds, saving money on the cassette is a case of “penny wise, pound foolish”. The Garbaruk chain ring I linked to above will also work for a 12-speed drivetrain.
That is what I did for my first true gravel bike…I upgraded my road bike to electronic shifting and took the Ultegra mech stuff from that bike and build up a Cervelo Aspero frame, but did it as a 1x. Used a 1x specific chainring and had zero issues with it…did many gravel races on it, including The Rift.
Yep. With 1X 11 speed there’s no real benefit to going electronic IMO.
What is the goal for doing this? You really feel that 52x11 is not enough gear for you? And why 1x? If this is a road race bike with short chainstays, I can’t imagine that 54x28 will be very satisfying to use. But maybe you live in pancake flat Florida and don’t need it anyway?
The cheap solution is to change your chainrings to 54/42, 53/39 or something of that nature and then save DI2 for your next bike.
Just to clarify, it is my TT bike i use for my IM events. Literally, where i live in australia, it is epic flat…so the races i pick, match my local terrain. I have not moved down to the smaller front chain ring in over 2 years.
So i will take the advice and change my front chain ring only and see how that works.
So should i move my rear cassette from a 28T to a 30T or greater if i change from a 52T to a 54T or greater clfront chain ring? Can the Ultegra RD8000 havdle that as the specs say it can only handle a 30T cassette…just basic numbers escape me with all this!
On flat terrain you don’t need a small chainring or even a wide cassette. I moved from a mountainous region to a hilly one to now a flat one. Even on my commuter with trailer and kid attached, I’m rarely in the lower half of the cassette (basically for getting going or to slowly ride on a sidewalk). And that is after replacing the 34-tooth chainring with a 38-tooth one.
On my road bike, I currently have a 10–33 cassette, and I think I could make do with 10–28 while simultaneously going for a harder chainring.
That depends: how often do you use the 28-tooth cog? If you use it and you don’t think you need an easier gear, going with an 11–30 cassette is the safe option if you upsize your chainring.
On a 12-speed bike you could get an even bigger cassette (e. g. 11–34) and get an even bigger chainring so that you have the same lowest gear, but gain a tad in efficiency.