I think the rate you get with "best rate" will be discounted and I wouldn't count on it going any lower - but you never know! I don't know about availability for your dates, but YC & BC were showing discounted rates when I logged in to Best Rate. I was just "playing", not really looking for anything, but I don't remember Poly coming up -- so that's why I said "don't know about Poly"!!
Sorry, didn't mean to confuse you!
BTW, you'll enjoy having an AP, even if room discounts aren't as great as they were in the past. The freedom to come and go at the parks; spend a little time or lots -- that's the REAL value of the AP!!!
Oh! And don't forget the DDE too! It saves 20% on many meals!