I think IGG's tendency to create a small "mini-game" at the start, is because the card itself helps fuel the war. The very mechanics of the card are built to buy more IGG. IGG creates a "rush" game, because the card aids the rush.
Knit's cards don't have that mechanic in them, also Contagion is worth 6, so the war would start later and would come at the price of not reaching for gold. I would say that the "penalty" of two coppers though, may not be steep enough to spread a -2VP card around. Only because, there are times that two coppers is actually a REALLY useful benefit, in which case, the war for Contagion rages and would dominate the game.
I think you'll find in play testing that Contagion would warp the board, but for reasons that are subtly different than what rink says (or at least for not the EXACT same reason as IGG does). I have a similar card in my variant, which I have play tested about 20 times now. It spreads a curse for purchase, at 5$, but provides less utility than IGG. That card CAN warp the board and sometimes it cannot. It really depends on how the board is set up. When it does warp the board, it becomes the exact war rink suggests. When it doesn't, its because the card itself doesn't accelerate the war, (it actually slows it down), so the "rush" factor isn't there. I've had games where the opponent who rushes my card has LOST. This pleased me, look for that same tendency when you play test.
When you test Contagion (and you really, really must test it, all speculation on a thread like this, is pure speculation, no matter how strong a player doing the speculation is), watch the velocity of the card and in particular, see if there are times the card is NEVER purchased or is only purchased a few times. If 90% of the time, a game with Contagion not only clears the pile, but also forces a war to obtain it, you may want to consider tweaking the card.
Please note these comments come from a very inexperienced player, but now has 54 games under his belt of testing his own expansion, (with one of the cards in the set, somewhat similar to Contagion). So my advice here should be taken with large grains of salt.