Source? POE Wiki contradicts what you're saying and what you're saying doesn't even make sense. There IS diminishing return on adding more and more IIR, but 300% is 300%.
Example, assuming GGG does it like every other game does it, let's say a mob can drop from a pool of 10 items and only drops 2 items. You have a 20% chance of getting the base type you want to drop, and then a 1% chance of it being a unique, so in reality a 0.2% chance on kill of it being the unique you want. 100% IIR will double the chance for items to be magic, rare or unique. Following the same example, 10 item pool and 2 drop. Same 20% chance of the base item type but now you have a 2% chance of it being unique, or a 0.4% chance on kill of it being the exact unique you want. 200% triples, 300% quadruples. It doesn't make the drop chance astronomical though.. even at 300% IIR you're still looking at a 0.8% chance of being the unique you want.
This is why IIQ is so great. 10 item pool and the mob normally drops 2 items. With 10% IIQ it now drops 3 items. Now you have a 30% chance of getting the base type you want, with 300% IIR you have a 4% chance of it being a unique so a 1.2% chance (math may be wrong doing it all in my head) of getting the unique you want. That 10% IIQ actually raised your chances of getting a specific unique by 50% (0.8% -> 1.2%), while tacking on another 100% IIR onto your 300% only nets a 25% increase (0.8% -> 1.0%). That's where the diminishing returns come in. Your first 100% IIR is a 100% increase. Your second 100% is only a 50% increase to what you had but it's still a 200% increase in drops, going to 300% is only a 33% increase and so on.