Have you considered how AI handles things and what would happen if AI had infinite MP?
I wouldn't change that, but I 'd be interested to see what happens - playing as Finland with 100 divisions against 1000 soviet ones hmmm...
Germany will have some 1000 too though.
Anyway, it'd be easier for you to cheat by adding manpower by event through the f12 panel. some +75 or the recruits rally to the flag event fired annually, for example.
How will you feel as a Finn gamer playing a 3.7 million nation with 50+ divisions?
Slow manpower growth is fine for me, but in Iron Cross, the growth Finland has is just too little. In the Darkest Hour, you have the ability to mobilize. In HOI2, the growth is a lot faster. But in Iron Cross, it's simply too little. The year is now 1941 and I've been able to construct one single division, and I have banished one earlier. If my existing troops have attrition, that alone is something my manpower growth cannot sustain. Building industry also drains manpower, because I must build one because there's not much IC to be used in the beginning. I also must research technologies with only one tech-team, and increasing IC and manpower growth are the main goal. I can't research much of land-doctrines or military technologies. If the Winter War event had fired, which I assume will happen usually, my 12 divisions couldn't do anything against Soviet Union. I'm not sure would I get some additional event that would grant me manpower, but even then, the amount of IC I have couldn't build much at a time. Well, at least if I had spend time researching military stuff instead of industry, since now, 29 IC for Finland is not bad at all really.
Adding manpower only to Finland would feel a lot like cheating. Therefore, I'm asking that would it drastically affect the game if manpower growth was like twice as fast? Would 2000 manpower instead of 1000 really affect the main powers? And if it does, would it break the game or make it just more random

One thing I've always disliked in HOI games is the fact how limited they are. It's only about WWII and scripted events. There's not much dynamic stuff going on, and playing as minor powers have very little stuff to do.
As a side note, the situation in Europe is currently quite interesting. Germany started their war against Poland in 1939, and in the gates of Warsaw, Poland started to push Germans back, and then France attacked. Soviet Union has annexed the Baltics, but didn't start the Winter War even though I refused their territorial demands.