Change of gearing - big difference?

My bikes vary between:

50/34 - 11/32

52/36 - 11/32

52/36 - 11/28

I barely notice.

Admittedly I'm a climber, but the only thing I notice between my bikes is having to get out of the saddle for a couple of really steep bits which I can spin through on the other bike.  

Big difference, but not as much as the numbers suggest. I made a similar step a couple of years ago, moving from a decent alu bike, to a carbon 3kg lighter. I did the maths, 34/32 to 36/28 loses me 21%, and 3kg reduced my all up weight by around 4%, leaving a 17% numerical difference. Given that the steps between your bottom gears is about 12%, the new setup effectively raises you by a gear and a half. On the road, it doesn't feel that much to me. Maybe it's the stiffer frame transmitting more power to the road instead of twisting the frame twice per rotation, or maybe it's the psychological advantage of riding a nice bike. I still ride both bikes, but on a tough ride I'd take the new bike every time, despite its higher gearing.

Use Sheldon Brown's gear ratio calculator. We don't know what gears you curently use, but you do, so by comparing ratios between the two sets you'll have a good idea what to expect.

http://www.sheldonbrown.com/gear-calc.html

Exactly the same as what I have - compact & 11/32 on the winter bike, 52/36 & 11/28 on the best bike.

Believe me you will notice at both ends of the scale, both your lowest gear for the hills & the larger big chainring for crusing along.

That said I like to try & use bigger gears in the summer when the weather is better & lower gears for the winter.

Switching from having a low gear of 34:32 to a low gear of 36:28 is a massive difference.

Regardless of the loss of 1.5kg, you'll notice it'll make hills much harder, it'll be like losing the 3/4 easiest gears on your current bike.

I'd suggest buying a larger cassette and matching derailleur.

I'd go a halfway house on this.  I had exactly the same issue but ended up only needing to put an 11-30 on the nicer / ligher bike running the 52/36 crankset - the 1.5kg weight was the difference up the hills so I just end up climbing (and cruising) faster

yes you will notice a 36t chainring compared to 34t. Fit a 11-34 cassette to offset the loss of the smaller chain ring. Alternatively fit a 34t inner chainring. It will work fine, iv e seen 53/34 set-up work perfectly

Try your current bike on 34*25, which is near enough 36*28 on the new bike. if you struggle then you will need to fit 11-34 to return the same easy gear ratio

FWIW - the 6800 GS RD will take an HG-x00 11-34 without any issues. Fitted a HG-800 and it didn't even need an adjust on the b-tension screw (new chain was fitted at the time)

That's a pretty big difference. Try your hardest local climbs using the 26 or 27 cog, as that's the rough equivalent of 36/28.

Find out whether the cage on the new bike is SS or GS - if GS you can go up to a 32 cassette (Ultegra 6800) or 34 (R8000), but might need a new chain. 

Do you ever max out 50/11 on descents/flat?

You might also take a while to adjust to the right cruising combination, I'm very used to 50/34 and found I was falling between combos when I ran a CX 46/36 chainset. Constant swapping of cogs around 18mph average was annoying for a while. 

On the plus side, if you're handy with a spanner all this is changeable for not much outlay. Sell/swap the group and fit the bits yourself, couple of hours work tops.

How often do you use your current lowest gear? Try going up the hills in the next lowest and see how that feels. Plenty of online gear charts (Google html gear calculator) for my preferred. New ultegra can take 34 tooth cassette so swap cassette, derailleur and chain and you're nearly back where you were for ~£100