Sorry to tell you but you are missing out one thing... also all other buildings profit from construction bonuses it does not matter...they are cutting costs as long as you build.
What do you buidl if your not buidling infra? Additional civs!
Taking rheinland as example:
The thing is i can build 2 infra in the same time i can build 1 civ at game start, but then i have 1 more cic available to build more stuff, while the extra infra first has to pay of, this will happen exactly after i did build 10 civs on the lvl 10 as the speed diffrence is 11,11%, in the same time in a lvl 8 i will build 9 civs so now both have 10 extra civs.
So those 5 extra slots in 43 in rheinland are worth half a civ, add half a civ for the second place in Niederschlesien and 0,7 for the moselland this is all you get, if you put refinerys its worth together 2 additional civs until 43 you can build in the same time not more.
Same goes for the UK.
Btw. Autarky makes it even worse as it doesnt count for infra only for civs, so if you go for infra in the few states I mentioned in the last psot do it early!
Just went ingame, building 10 civ in a lvl 10 infra with construciton IV, Autakry Free trade, schacht takes 420 days building 9 of them in a lvl 8 infra takes 414 days, so even after 10 civs your still not even...on costruction 3 its 440 vs 441 hell yeah you safed 1 day...(still not counting the 44 days i had this 1 civ in the lvl 8 earlier that brought me extra cic output)
I stick to my word, just forget about it.