I feel like the punishment/weight ratio is rather logarythmical, as in it increases with route steepness but not in a linear way - you'll start noticing it on really steep gradients and with tiredness.
If you have a somewhat decent shape and don't ride over too much steep gradients I'll say just keep the steel fork. It's cheaper if you just lose a kilo yourself than in your bike. And depending on the builder, steel frames and forks can feel as flexy if not even more than carbon.