With our add-ons in the past we were able to get same rate as Disney was offering to the new purchases. Saw the same reference you did as to higher add-on percentage rate and then later that person reported that they could get it for the lower rate. However, I just don't know the current status. Nevertheless, I believe it likely you will get Disney's best rate if buying 150 points or more because that is the same as buying a new contract.
My view is to buy as many points as you possibly can off the bat. You can set up any new purchase as the purchase of more than one contract, e.g., two 200 point contracts, as that will potentially make later resale (if ever needed) more likely as lower point rather than higher point contracts sell faster in the resale market (although you really don't need to make it 150 point increments--most contracts in the 200 to 300 point range have sold in resale fairly quickly).
The reason (other than future price hikes) to buy as much as you can is that eventually, and probably sooner than later, the resort you are buying is going to sell out and if that is the resort you want to stay at most of the time you will want home resort reservation advantage with as many points as you ever want to use. We own at BWV and have the number of points we want for the forseeable future. It took over 4 years for BWV to sell out and thus we had no problem with add-ons. I believe there are many who wished they had purchased more at BWV because it is difficult to get ressies there many times of the year with only the 7 month window. VWL is going to sell out later this or early next year. If that is your desired resort you may later regret not buying now the total points you believe you may need for the long term future.