Well I found that the stock coils on my r33 died after I went above 12psi, well they were fine until I hit ~4500rpm and then would cough and splutter. I whacked in a set of these and didnt have a problem after that.
There are a lot of different opinions about stock coils vs splitfires, but I have had only good experiances with splitfires.
My 2cents, dont buy them until you need them. And make sure you identify the coils specifically as a problem before you buy splitfires as what seems to be coils might be another problem, and buy splitfires might cover-up other issues, but then again I am by no means a mechanic, nor do I know much about cars or motors, just typing from experiance.