If you boil the canned kit, it will get darker which might not be want you want if you're making a lighter colored beer. Other than that and wasting some fuel, I don't think there is any harm boiling it can do. Its just not necessary.
If you make a Cooper's kit, by all means follow the instructions. If you just have to do things your own way, you might try a different yeast, maybe try 2 cans at once and not add the extra kilo of fermentables.
Some people recommend adding 2#s of DME or LME instead of the sugar, I would recommend not doing that unless you are planning on adding some hops as well. It will work out ok for a darker kit, but if you try and make like the Canadian Ale, or maybe one of the Lagers - don't go nuts with the DME, if you want a maltier beer - use two cans and skip the kilo.
If you really want to make a good Cooper's beer from a no-boil kit, try the Lager, European Lager, and Heritage Lager and follow the instructions that come with the kit. For the regular Lager, it comes with an Ale yeast so if you can ferment in the 50s you might try an inexpensive Lager yeast like S-23.
Lately what I like to do is make a big All Grain beer, and then usually I can get another couple of gallons of wort from the grain after I'm done mashing. I use that leftover wort for the water on the canned kit, and skip adding the kilo.
You don't have to follow the instructions precisely, but I seriously believe that you are much better off when you do. The first few canned kits I made I followed the advice from online like on this site, and the results were so-so. Its not any wonder that the people who give you that advice about the canned kits look down on them and think they don't make good beer. But if you follow the instructions you'll make something really good.