To be honest, growth is not very important... except that you can use it to get troops. Even in that regard, a growth over 20 is not very important until you cannot find targets to hit to raise your GDP (like my current situation) because raising your GDP through war is much quicker than through growth. A new player with 114 GDP and an insane 39 growth (max sustainable without spending more cash each turn) that DIDN'T get hit by someone would take around 35 or 36 turns to get to my current GDP level. I reached this level through war in only 26 turns, and even then, that includes the turns that I completely did stupid things, as well as the fact that I've sent about 30k troops and about 1.5 million cash to different people. War is faster until you cannot war any more, and once you get to that point, Leap Forward will get you to a high growth rather quickly, considering you can do it 10+ times each turn.
edit: Also troop costs are very easy to find out. If you are smart (I've found that every time I hit leap forward, if I suceed and I spam it quickly I will get 4 or 5 successes in a row. If I fail, I wait a few minutes, try again, and repeat until I succeed) you can get 5 growth for 600k. It will cost at least 500k for 5 growth, but the average is more 700k. If you have untapped manpower taking 10k troops out will lower you slightly, but it will go right back to untapped by the next turn. Selling troops only nets a profit if you are just out of being able to use humanitarian aid because the aid only costs 75k, so it will cost 375k for 10k troops, and you will be making 325k out of it.