Brake calliper upgrade query

You don't need to change calipers. Shimano themselves have stated that their top three groupsets are identical in functionality but differ in weight. Brake pads will make the most noticeable difference. Get the best you can for your type of rim.

I upgraded from Tiagra to Ultegra and would not go back. I changed after descending the Gospel Pass with friends who were enjoying the descent whilst I was struggling. I had already upgraded pads to Swissstop, but that was not enough. Still use Swissstop though.

I cjanged from 105 to Ultegra and the difference was amazing, the feel and power was very much improved.  The problem was that I then had to upgrade both of my other bikes.  Put the cost of a new pair of pads towards the cost of new calipers but be careful that the new calipers have the same cable pull as the old ones you are replacing.

There won't be any real noticeable difference between performance on 5800 calipers Vs R8000. I have both 105, Ultegra and DuraAce calipers and performance is more dependent on cable set up, wheel rims (Exalith are excellent!) and brake pad choice.

You won't notice any real difference between the two, Pads are what can make a difference, but I've been more than happy with standard Ultegra pads

Best post of yours I have ever read.

IMO I don't think it's necessary, just make sure to set up as normal and you'll be good to go. Potentially a new set of pads but scrub in beforehand.

I've descended in the Alps on Avid Digit 3 V brakes, rear was a ceramic rim, front a standard alloy job, it was dry mind but certainly not high end brakes and I was at around 50mph in places on my hybrid. BITD and when I was a lot younger I used to descend on Weinman side pulls with leather pads on Alesa alloy rims (top speed was about 55mph on a Raleigh Reynolds 501 Sports tourer). I'm nothing special so you should be just fine.

That said and not knowing how much you've ridden/descended or used the Exaliths's but the trick with them is bedding in the rims as some people seem to go through the first set of pads really quickly and then everything seems to settle down. I've got a Haute Route set but lightly used and not on anything steep as yet though a mate who did Raid Alpine had some Exalith's and he was fine even in the sopping wet coming off Col de L'Iseran, not sure what calipers he had but IME I really don't think it matters that much.

What does with any riding and particularly descending, making sure you are braking well before your speed gets out of hand, if you're not familiar with the route then simply letting the bike run really is not a good idea if you can't actually see ahead much (think of the Highway Code rule 126 and you can't go too far wrong). 

people want the buzz of going fast and descending but not taking risks you don't need to (it's not a race) is more likely to get you down in one piece, I've known of too many who simply went too fast, panicked and then crashed, they didn't have enough thinking/braking time in hand and again the actual brake calipers became pretty irrelevant to the argument.

Getting your line right (and of course speed) well ahead of twists, turns, junctions etc should be high up the list of your planning/advanced thinking.

Sorry if that seems a bit preachy, but again, don't know your experience/background.

Have a great time.