I have done a few of these Coopers kits including the real ale. My opinion is that there is a limit of how much sugar/dextrose you can put in before the flavour starts getting a bit nasty.
I plugged what you have into Beersmith to get you a rough idea of what to expect.
If you brew 23 litres with the can and brew enhancer you are looking at about 1.038 for 3.8% ABV.
If you brew 23 litres and add 200g extra dextrose you get 1.041 for 4.4% ABV.
If you brew 23 litres and add 500g extra dextrose you get 1.046 and 5.2%. In my opinion you might be degrading the flavours at this point.
Another way to increase the ABV is to reduce the volume a little.
If you brew 20 litres with the can and brew enhancer you are looking at about 1.043 for 4.4% ABV.
If you brew 20 litres and add 200g extra dextrose you get 1.047 for 5% ABV.
If you brew 20 litres and add 500g extra dextrose you get 1.053 and 6%.
This will increase the hop bitterness a little but you should still be ok.
This is all just an estimate, but hopefully gives you some idea of what to expect. Personally I think you are best off with one of the 4.4% options. I'd take a nice 4% beer over a not-so-nice 5% one every time (I say this having tinkered with Coopers kits and made several of them higher ABV but still worse than if I'd just stuck to the recipe).
Also, the Coopers recipe will probably say to ferment at 21-27c. I'm not sure why they say this, you will get better results at 18-20c every time.