The problem with the Throughput modifier is that it only improves the efficiency of workforce, as increase in throughput increases both output and input but keeps workforce the same. But it doesn't improve the actual efficiency of the building as much due to most costs being input goods. So even a good strong 25% throughput bonus probably is only 5% better than default when you consider other factors. You have to stack the modifier crazy high to have significant advantage over someone else.
There are a number of points though:
-you dont have to work with PM's that require input goods, at which point throughput fully applies to labor cost alone. As such, it also differentiates itselfs as a potential high yield "labor intensive industry" in comparison to a "capital intensive industry" where the latter in this game would be modeled by requiring more advanced inputs as in part "leasing its machinery/tools". The game also features circumstances where there are significantly lower labor costs afterall, so you can create some pretty extreme profit margins on labor intensive industries with throughput.
When you apply 100% throughput you double production but also double to input requirement. If said input requirement is 80% of the cost and labor is 20% of the cost you only make a 10% cost reduction trough throughput, will also increasing output and output per worker. If there is no input requirement like with early plantations for example then the cost reduction of 100% throughput is 50%, thats a lot more. It is notable that plantations can often get up to 150% throughput at which point they have the output of a nominal single plantation with upgrades pm. Such labor intensive methods quite favor country's that can fall back onto having significantly lower labor costs or who are utilizing slaves for example.
Secondly: "Throughput stacks" Assume just as example that 80% of clothes production lies in the cost of cotton and dye, but i have put a 150% throughput on cotton and dye and managed to make them 50% cheaper as a result while making their labor cost 66% less. Now i have also applied 100% throughput to these clothes factory, nominally that would only half its labor cost but since we also managed to half its input costs well we managed to half all the cost. This throughput stacking can also circle around in the case of iron/coal, steel and then tools back to iron and coal, the cost reductions affect industry down the line who affect more industry down the line and when the tools are hence 50% cheaper because all the rest was so much cheaper we come back to the miens using cheaper tools and providing even cheaper iron for cheaper steel and tools production.
Another thing to consider is that low labor costs often translate in competitive trade advantages or high dividend yields or both. This can have myriad function like for example for the purpose of trade and gaining tariffs or for the purpose of buffing the investment pool trough a x3 x2 x5 modifiers. Yes buff the investment pool to the point that it has the capacity of a country with x30 times your GDP, you would think Paradox have become careful about multiplicative modifiers but here we are. For Small country's highly profitable throughput industry's coupled with low labor costs can be something stronk indeed.
So i think you need to rethink what throughput can do for you with a few of these considerations in mind. Throughput can be quite powerfull for a varried number of strategies and especially when it acts in a chain from agricultural input for example to industrial output. If you have a great farming province that can produce coton, dyes and silk and can have very cheap labor then its not impossible to set up a center of clothes production there that can easily drop the rpice under half the market price and still be quite profitable, by the 70'/80' you can typically start to dump loads of clothes on various markets especially at that price.
Indeed, rethink even for many country's "if you albeit want to use upgraded PM's", i know it sounds silly and its usually no gain on the really longer term but for various reasons and for various buildings you can do some silly things with it. Like the concept for example of "paperless bureaucracy", which is intentionally running oversized bureaucracy's withought paper in part to make use of having more throughput on said bureaucracy's, but also knowing that said buildings PM's have relative few bureaucracy gain in contrast to tax capacity gain per level so that if your purpose is to produce bureaucracy that is cheaper per point of bureaucracy you can often make a gain by going paperless on them and just build more, and its good in that sense that nowadays they are cheap.
