Making custom Cassette

I haven't got around to combining my 5800 12-25 and 11-28 cassettes to make 12-28 with the missing 16T I need so much. I'm also on compact and I full agree with your approach. Small cost for ultimate convenience. I use a 6800 14-28 cassette on my turbo trainer, it's wonderful!

I'm not a racer but I'm quicker and more powerful than most. I use 11T and 12T a bit but I don't think I would miss them much, i think losing the 13T by using 14-28 on the road might be a step too far though.

I think the A/B/C cogs differ in where the shift ramps are, I haven't compared 16A with 16B yet to confirm. I think it would just mean a less perfect shift when you quickly skip across a few gears that go across A/B/C.

How about a CS 5800 105 cassette with 12-25 and use some of the lower ratios to get what you need using the 20-22 & 25-28-32 from your current cassette? not too expensive and should work with your Ultegra bits. You could then have 12-13-14-15-16-18-20-22-25-28-32

http://si.shimano.com/pdfs/dm/DM-CS0004-04-ENG.pdf 

Another shout for custom Miche cassettes. I have 13 or 14 Miche sprockets which I combine to give:

13-14-15-16-17-18-19-20-21-23-26 for flat riding

and

14-15-16-17-18-19-21-23-26-29-32 for the mountains

(admittedly, I did whack a Centaur 11-32 on my best bike for my last Euro trip, because I couldn't be arsed to move the Miche sprockets off another bike)

Personally, I wouldn't create a custom cassette from OEM clusters unless you have them in the spares box. I wouldn't change your chainrings either; it doesn't sound like you want more top-end, and 6800 items aren't exactly cheap.

Separate sprockets will also allow you to tune your ratios for very little additional cost, should you discover when riding that you miss that nT sprocket.

It seems odd to go for 53/14 as your top gear. That’s lower than 50/12 by quite a long way. At the other end 39/32 is a marginally higher gear than 34/28. Why not stick with a compact and go for a 12-28? You could do 12, 13, 14, 15, 16, 17, 18, 20, 22, 25, 28. 50/18 is lower than 53/19, so your single steps still work out.

I've combined CS6800 11-32 and a 14-28. to make a 14-32 Cass. one step from 14 to 19 in single teeth jumps. if you then change your chainring for a 53/39, swap out the 39t ring for a 34t you will have a fantastic range with small steps

Some places sell totally custom cassettes, where you choose all the sprockets yourself and they send them to you. I know miche do them for Campag, not sure if they do them for shimano/sram as well.

It will shift fine. The sprockets used are the same between clusters. 

What you suggest would be fine. But here is another suggestion.... don't swap the cassette, just get a 39T inner chainring. It will satisfy 99% of your flat riding needs with nicely spaced 12,13,14T rear cogs. Save the big ring for sprints and descents.

You could mix and match the first few ratios because the cogs are seperate but the last few are riveted together. It was much easier back in the day when they wer e held together with teeny hex-headed bolts. You should be fine though, the 15 and 16 tooth zone is well within the seperate cog zone.