Judging from past experiences uprated HT leads are only any use on high boost turbo applications in conjunction with colder plugs to maintain spark as high boost pressures. On standard plugs and leads it has been known to blow the spark out.
For that reason a higher current needs to travel down the HT leads to the plugs to give a brighter more powerful spark.
On an NA car never really noticed any difference, car manufacturers usually put plug leads on a car that will last 50K miles or above.
Magnicor's life time guarantee is good but if they ever fail they have to be sent off for testing, leaving your car not running until they decifer the problem. In which time you'd buy a new set or put the standard ones back on anyways.