HoI1 used IC to fuel research, which is the incorrect approach IMO. The only thing I liked about HoI1 is that you could scale up your research by brute forcing IC, whereas it's quite difficult to scale your leadership up to further speed up research. That said you can accomplish focused speed increases by building things, so in a fashion it's quite similar, and more reflective of reality. Also because of the design of the HoI3 system you get much more free form results than in HoI1. Instead of picking a simple tank cannon for your tank and being stuck with it, you can actually make improvements over time, instead of simply researching 40mm+ tank gun and being stuck with it for the remainder of the game. It's a system that can work well into the future.
I often ended up with Panthers with 50LL guns in HoI1