They do know what the spring rate is. It is 10#. The fact that they are considering changing it, I don't think indicates that there is anything wrong with the 10#.
You need to understand that in the M&P design you do not need a heavy recoil spring to keep the barrel in battery during ignition. The only thing the heavier spring would is slow the slide's velocity down a bit. Do they need to slow its velocity more? NO, slowing the slide more would reduce the slides impact on the frame but the current combination has already demonstrated exceptional durability . So, I don't think changing the spring weight is really going increase it's durability much, maybe they think they can get take it just a bit farther.
This is just my guess: :roll:
I think the current spring set up was intended to run both 9 and 40 and would be easier to make just a single assembly. But now with the 45 coming out they are going to have a Blue (10#) and a Green(13#). So if it will run on the same spring as the 45 and you have to produce a new assembly anyway why not give it a try. I fit gets a little more durability, bonus.
I could be wrong all together and this is just evolution in action, they found it just works better with the 13# 8)
As far as factory replacements just give them a call ask for part #279740000 $6.36
As far as lighter springs IMSI makes an 11# spring which if the 13# IMSI feels like stock I may deduce that the 11# may feel lighter. But again I may be in left field (been there before and the grass was nice
)
Dan