In order to combat this problem, the 665p will introduce the dynamic SLC cache feature. When the drive is at 50%, it'll have a maximum cache of 280GB for the 2TB variant and 140GB for the 1TB variant. Once the SSD breaks the halfway mark, the cache starts to decreases until it reaches a fix amount, which is 24GB for the 2TB model and 12GB for the 1TB model.
This sounds like what the 660p already does. The dynamic cache starts at 140GB for the 1TB model, and works its way down to 12GB when 75% full, and the 2TB model doubles that, with both matching the numbers listed here. I believe the 660p starts reducing its cache around the 25% mark though, so perhaps they adjusted the curve for the new model to maintain more of that cache when a larger portion of the drive is filled.
A $160 MSRP for the 1TB 665p doesn't seem all that attractive when the 1TB 660p has been widely available for under $100 for the last 6 months though. Based on these numbers, the improvements seem quite minor, after all, and probably not worth paying substantially more for.