Ok I'd like to add my 2c.
First, are you ok with spending $2k on tires every other month? Because that's exactly what you're going to do with the P85+ and those low profile rear tires. If you don't blow them out on every pothole you hit, you'll certainly burn through the rubber in no time(5-7k). And if you go blow them pot on a pothole or other road debris, you'll probably also be replacing the wheel too. Add another $1500 or more for that. If you have unlimited funds though, then this probably isn't a problem for you. Oh, unless your time is important. Because you'll be spending a lot of time in tow trucks.
Also, if you live in an area that sees sub-40 degree temperature, you DO NOT WANT the 21s whatsoever.
That being said,
I just replaced my OEM 19" goodyear's on my P85 with 255/45ZR19 Michelin Pilot Sport A/S 3 and HOLY CRAP it makes a MASSIVE difference. The car no longer slips under heavy accel. All that floaty feeling is completely gone. Stomp on it and it just rockets. It feels planted like none other, the grip is amazing. On top of that, it is MUCH quieter than the goodyears ever were. The ride is much incredibly smoother too. I'm amazed at the feeling now. It feels like a brand new car and a totally different driving experience. I'm literally shocked at the different these Pilot Sport A/S 3's have over those goodyears. I had no idea swapping the tires like that could have such a massive effect on the driving experience. IMO there is absolutely no need for the + package or summer tires or those 21s. If you're looking for grip and traction and performance and smoothness and quiet ride while making NONE of the sacrifices and issues or the maintenance costs of the 21s, then these 19" Michelin's are the way to go. Even though they are technically an all-season tire, these Pilot Sport A/S 3's are rated even higher than summer performance tires are with regards to dry traction. On top of that they are also at the top of the charts for wet traction too. Really you get the best of ALL worlds here. Stick them on TSportline TST Turbine wheels and keep that factory look going too. This is really the way to go. Oh, they are lighter too aka lower unsprung weight.
best aftermarket investment for the MS by far. save yourself $10k and just get the regular P85 and stick these tires on instead.