I was able to get to below invoice on my 2017 Forester that I preordered when they were still clearing out 2016s. Subaru doesn't typically over-produce like many of the bigger makes do so there's usually not a ton of inventory to clear out. They started the 2017 Legacy incentives at least a month ago and the 2018s won't be here for at least another 3-4 months.
You should be able to get with a couple of % of invoice on your 2018 no matter when you order it.