I think the answer to that somewhat depends on the infrastructure decay rate. Will that be based on population to an extent? Should the commerce spread out in this manner be decimalized and then equally divided or should it be divvied up by population with larger cities getting more attention?
I'd think units would all have a natural small drag they would offer the infrastructure as well (representing added use, wear and tear if nothing else.) Though workers could be a good set of units to be able to develop a passive bonus source - particularly with their new promoting capabilities.
One thing they did with the mod that brought this up was to base (I think) worker build rates and improvement upgrade rates on the investment slider and improvements all had numerous upgrade layers. If we did what you're suggesting, I'd suggest having improvements expanded into more 'stages' that can then grow or retract based on the current Infrastructure levels on those tiles.
So commerce from the slider would also have to split off into every individual tile as well. And as a slider, the 'limit' is simply 100%. It could also impact GP rates to let the property fall too low - actually, via autobuildings we can play on just about every kind of detail in the game.
Of course, it could become ridiculously overdeveloped by a player who runs with that slider too high for too long such that the infrastructure property values are 'off the charts'. Because there'd be a limit to how much benefit you could derive and a limit to how many penalties you could collect. But if done right, it would have to be focused on with an eye for balance to get the best functioning empire. They stated on their page that without running the slider at all and just going for research, you'd end up actually hindering your research. I liked that concept.