15% seems like as good a deal as you are likely to get. I always try and get things for 10%-15% discount. half the time just asking for it I get it .. not boat stuff mind you im just starting to buy boat stuff. and probably you should take the deal when it shows.
I bought my autopilot (Raymarine SPX5 wheel pilot) at last year's Defender sale in April. The upgraded version (Raymarine EV-100) is being sold by Defender right now for the same price as what I paid last spring: $1,339.49
I looked for a 6 month period and don't think you'll see it cheaper than that.
Since today is Buy Nothing Day I guess I'd recommend buying it this weekend.