New bike dilemma

You are not racing as a pro or trying to win for fun at sportives, so on a rational basis it will be a complete waste of money. You'll feel no difference on flat roads and I even doubt you'll feel a difference going uphill. For the pro peloton a couple of kilos can make the difference. For youi its a non-issue. Performance is not important to you, you don't race, you're not a pro.

But if you like a new bike because it makes you feel better when riding, just the pleasure of owning the best, then go a head.

And don't underestimate the demonstrable improvement in speed you will get from tighter fitting/less flappy clothes, and getting into a more aero position when you are pressing on... flatter back, hands on the hoods and forearms on the bars parallel to the ground.  To sustain this may well require practice and core work off the bike.

All that said, if you just really want a shiny new bike then go for it!  That Orro is a thing of beauty...

2 kilos saved wont do anything for flat speed and given your health condition, you arent going to get the best out of buying a slighlty lighter bike for going uphills.  Spend money if you're inclined on Wheels and tyres-actually just tyres.  A quality set of performance tyres will make the bike ride much nicer and a bit faster.  

An electric bike will be much spendy but give you rocket power for uphills

Proffesional reviews put the Datum 20 at 9.29kg overall weight with 105 groupset so I presume that means you could very easily get it down to 8 something by spending a few hundred on upgrades (be it swapping out most or all of the 105 for Ultegra, or probably even better as Simon and others suggested, switching wheels and tires not just to shave on weight slightly but to add in a more overall sense to a faster and better ride quality). Although it seems on the very heavy side for a modern carbon bike, I would consider any bike in the 8 something category to be light? Obviously not super light (It's easy to find 7 something kg Carbon bikes from reputable manufacturers for the same price as Datum or probably even less) but still into the light sphere nonetheless.  I somewhat doubt shaving off a kg or so via buying a lighter bike would necessarily offer a perception of much improvement. I mean this Datum is highly reviewed and given it's quite steep price and the fact it's notably heavy for a modern carbon bike, one would guess it must in turn be somewhat special on it's own terms.

Sounds like you are very comfy on it but just want to go a little faster. Personally i'd follow the other's advice and upgrade to lighter and better wheels and tires, then work away on my training/ good positioning/saddle at exact optimum height etc. A bike a little under 9kg would surely be no slouch that could hold you back in getting much much better and faster with some casual training!

Furthermore you'd be shelling out only a few hundreds as opposed to at least 3 times as much to buy a slightly lighter bike. If I was intent on a new bike, I'd probably look to get just the frameset (a considerably lighter one than the Datum) throw on my current parts and flip the Datum frame on ebay or classifieds (being a somewhat expensive and prestigious bike, maybe the frame would have a fairly good resale value?.. as far as frames go) Trouble there is that typically (at least if buying new) frames are almost always overpriced and if i was to go that route and find my new bike was less comfy and not noticably better i'd be completely gutted.

If you're happy with the comfort I'd be loathe to change the frameset, but if you do pay close attention to stack and reach so you can achieve a similar position. Potentially if this is going to be your last bike maybe consider that you'll need to make your position less aggressive as you get older. Maybe a custom titanium frame to the same dimensions if you intend to keep it forever?
Personally I'd keep the frameset and go with a hand-built performance wheelset, new tyres, and maybe a groupset upgrade (you might find good deals on eTap come the Spring). Should be able to get the Datum below 10kg then.

You could spend the bike upgrade money on upgrading your existing bike, there's some cracking deals about.. how about Ultegra Di2 and a new set of carbon wheels.. that'd give you some pep in your pedals, no doubt drop a load of weight and you'll keep your same comfortable position/bike. I found carbon bars made a big difference in comfort. 

I only mention Di2 as I still get enjoyment from the little buzz buzz sounds it makes

The Datum is highly rated and the comfort you like is something you might not replicate on a model with different geometry. IMVHO even if you buy a 7 or 8kg bike it might initially feel nicer but won't transform your riding experience but will make a huge dent in your wallet.

I'd invest some money in nice wheels (Hunt, Cycle Clinic, DCR et al), light and fast-rolling tyres and some good quality clothing, if you haven't done so already.

Road.cc's newly updated road tyre buying guide

You will then be well kitted out and still have plenty of money to spend on cake going places and enjoying it.