the key to a good beer is balance- a balance of hops, malt, alcohol.
If you change one of those, without adjusting for the others, you can have an unbalanced beer that doesn't really taste that great. As an analogy, say you're making spaghetti sauce but you only have enough for 6 and want to feed 12. Adding double the tomato sauce will give you enough for 12, but it won't be the same sauce with the same flavor. You need more seasonings, too, for the bigger batch. Just adding the liquid won't give you the same sauce.
It's sort of the same way when increasing the ABV. You can safely boost the ABV a little bit without much effect, but if you add more malt or sugar, but not more hops, it won't be a beer with the same flavor and depth. So balance is hugely important.
I guess my best advice is if you want a 5.5% beer, start with a recipe for a 5.5% beer at the beginning. It's much harder to make a 5.5% beer out of 4% beer than you'd think.