the biggest problem with trying to prevent heat radiating from the object is thermodynamics. Basically, delta-entropy >= 0. It can never be <0. Entropy is basically "heat" BTW.
So effectively, stealth in space like that, is going to be real hard. The other problem is that using a slingshot effect or an Oberth Maneuver (2 different things BTW) requires a lot of, well, "luck". You need everything lined up just right. A mass driver, railgun, or coilgun will be much easier to use.
The biggest problem with a projectile is that- beam weapons can travel MUCH faster, and missiles can be MUCH more accurate. Remember, in space combat, you are firing at where you opponent WILL be based on where they WERE. It's like leading a target in a gunfight in a dogfighting sim or an FPS game. Except on distances of millions of km and lightspeed lag involvement.
It is really difficult to get an iron ball (say, 100 kg, or around 220 lbs) to do city-killer levels of damage. Because no launching device will be perfectly efficient (and gravity slingshots&Oberth maneuvers won't give enough delta-v (which in this case is an increase)), you need more power to launch the weapon than the weapon does damage.
I actually was trying to make up a plausible&effective plasma weapon for an SF story I am working on. I wanted a plasma weapon because 1) plasma would be a cheap ammo type and 2) it would be something that would look cool (should I turn the book into a game) and something that would have LOTS of ammo for it. The problem I ran into is 1) plasma likes to equalize pressure with its surroundings. So I make the decision to use a plasma round that masses 1g and travels at 0.5c (50% lightspeed).
Sounds like a good idea doesn't it? For a game, I could just make the effect move slower (so it looks cool), and for the book, it allows me to get around plasma pressure equalization issues. The problem? 1g causes 1.39 TERAJOULES (re: nearly/slightly more than a ton of TNT!) of damage. This in itself is actually REALLY COOL, but not good because the weapon is supposed to take several shots to destroy an enemy. Where it starts getting worse is the fact that if the weapon is 99% efficient, it still takes more than 1.39 TJ to fire ONE SINGLE FREAKING SHOT!
A suggested velocity given by DoomBringer was 20,000 km/h. This works out to (1h=3600 s, 1km=1000m) 5555.56 m/s. Wow, we're really clocking now *in boring monotone*. The energy of a 100 kg mass of iron at (we'll say 5600 m/s for simplicity) is Ke=100kg times 0.5 times (5600^2). Which works out to Ke=1,568,000,000 J or about 1.6 gigajoules. A little less than a TLAM-C's 454 kg TNT charge. No city-killer there. We'll say 50% efficient railgun launcher, and we get a firing energy of nearly 3.2 GJ. If we say exactly 3.2 GJ firing energy, then we need to pump 3.2 FREAKING GIGAWATTS into the friggin' thing! That launcher, if in space, is going to show up FOR BILLIONS OF KM AROUND! There's simply no way to hide the thing.
Let's whip it up to oh, say, 20,000 METERS/s. Now we REALLY ARE clocking (literally. That's nearly 7% the speed of light (which is 21000 m/s for inquiring minds)). Now, even though we're going really fast, it is still hypervelocity, and not relativistic (yet). So, Ke is now= 2x10^10 J. This is, quite incidentally, the energy of an average lightning bolt (which is 48 tons TNT equivalent). Still not a city killer.
So let's REALLY up the ante and say we are going to shell this round out at 50% the speed of light. A new equation comes into play, for relativistic corrections, and now we have Ker= ((1/sqrt(1-P^2))-1)*M*9e16; where P=% speed of light (expressed decimally), M=mass (in kg), and 9e16=c squared. "sqrt"=square root.
Thus, we get a Ker=1.39x10^18 J. Which is a bit more than 239 Megatons. Now we have our citykilling 100kg iron slug. The only problem is, if we assume 50% efficiency for weapon launch, we need 2.78x10^18 J of energy to fire it. That energy for firing is nearly equivalent to 674 megatons of energy!
So, in reality ManSh00ter, to use a reasonably inexpensive slug as a city-killer weapon, you need what is sometimes termed an "R-Bomb", a weapon that moves at relativistic velocity. And that takes WAY too much energy to be effectively feasible from an economical standpoint (remember, you say that the weapon is cheap to employ).