If you have set the target a little higher for your car you cannot just buy a "good" coil.
A coil which is "good" in one envirionment may very well be "bad" in another environment.
You have to look at the whole system, as was indicated in some earlier posts.
For example, you buy a "high performance" coil. These require by nature high current. If your system/box/distributor cannot supply the current needed, the spark from your "high performance", expensive coil will be weak, and you have wasted your money.