This sounds very good, but as it solves one problem, it produces another. Changing the goods produced by an existing buildings does not sound very fair to me to those who have utilized them in their city design or FS balance.
I've seen a chapter 15 city boosted in scrolls but doesn't have any scroll factories. When I looked closer, they had 6 moonstone libraries and a bunch of endless scrolls with the appropriate other set pieces to maximize scroll generation. That produces a lot of scrolls with no cost in population or culture or of coins and supplies to keep production going. It that changed to silk for them, it would take quite a while for them to recover. Their city would be dysfunctional for probably over a month. Yes, it's an unorthodox setup, but is it fair to do that to them? I can see someone reducing their need to produce scrolls allowing them to produce more of their other boosted goods. So, this is an example that contradicts your 2nd pro of it penalizes someone boosted in scrolls.
I have a fairly large moonstone set specifically for the scrolls. It makes me a scroll provider. My T2 is crystal, so making it become T2+1 wouldn't create a change for me. If I was boosted in silk getting crystal instead of scrolls doesn't sound so bad, unless I'm in an FS that is overloaded in crystal production and has too little scroll production. This would exacerbate the problem of too much crystal and too little scrolls until people leave the FS and new ones are brought in. In this case it penalizes an FS that has found a way to adapt to the member load it has established.
It's not hard to imagine a decent number of people using the sets in this manner. Would this change be fair to them? Would you want this change if you were in those shoes?
I'd prefer to leave the Moonstone set as it is, but replace the set given by the spire to something new which is adjusted according to your boost. Although, I'd like to see it be boost -1 instead of +1. There are so many buildings already that are +1, let's go the other way for a change. I think that would give a better balance to the individual. I've got several things that produce scrolls. I'd like some silk, please. There have been discussions of changing what buildings are produced in the spire but they have seemed to devolve into trying to totally rebalance the goods. Just change the set to a different one every now and then. Eventually you end up with too many of any type of set from the spire, even if they are boost +1 or -1. The spire set needs to change every now and then if you want people to become more engaged in it. ... but that's another idea for another thread.