You asked for it, so don't complain.
Adding and infrastructure based multiplier to IC and/or ressources doesn't make sense from a mathematical, socio economic and gamebalancing standpoint.
It adds a new layer of oversimplification, wrong assumptions and bad math on top of an already oversimplified and problematic mechanism.
Infrastructure needs a rework, there are a lot of elements that are not properly tied into it, which falls imo in the bugfix category that should be adressed soon, but IC and ressources are not one of them.
There are already too many things handled and influenced by only one generic "infrastructure" value, but this idea makes the whole situation only worse.
It would help if we have independent road and railnetwork values, and a ressource system that is able to handle limited deposit sizes that can be used up, and a min/max output value for that deposits that is determined by a new province building.
But that is a major rework and not something you put in the game halfassed.
To develop about what ewphoenix wrote, I think that there is a confusion in AoD between infrastructure and industries.
I won't develop about the need to have a railroad system in place, but what I wrote below belongs to a wider reflexion.
When there is an industry, its output/productivity depends of the technological level. This is modeled in game, usually by 2% steps. This has nothing to do with the infrastructure level.
IRL, the level of infrastructure and the numbers of industries are related, but rising the infrastructure doesn't raise the productivity, it raises the number of industries that the network can supply.
The opposite is true. A cluster of industries can't be, if the level of infrastructure is too low. But having new industries allows to develop the level of infrastructure.
IMO, there should be a correlation between the number of industries and the level of infrastructure in a province.
Something like:
10% infra <-> 0-0 IC
20% infra <-> 0-1 IC
30% infra <-> 1-2 IC
40% infra <-> 2-3 IC
50% infra <-> 2-3 IC
60% infra <-> 3-5 IC
70% infra <-> 4-7 IC
80% infra <-> 6-9 IC
90% infra <-> 8-15 IC
100% infra <-> 13-...IC
These numbers are pure example, but show the main idea:
To increase the IC, it requires a minimal level of infrastructure, an higher IC allowing in turn to increase the level of infrastructure.