A more powerful version of a coil will typically have fewer windings of the same wire gauge. Alternately, it might have a similar number of windings of a larger diameter wire. The "power" of a coil is roughly inversely proportional to the total resistance. The lower the resistance, the higher the current passing through the windings and the more force the coil will deliver to the flipper bats.
Some vintage EMs just had notoriously weak flippers and owners desired to pep them up. Other EMs had wear or increased friction in the flipper mechanism, and people would install more powerful coils rather than fix the real issue.
The orange and yellow versions of the A-5141 are reasonable alternatives to consider. If you have rebuilt the flippers and have properly adjusted EOS switches and your flippers are still weak, consider trying an orange dot. If you install more powerful coils on an older Gottlieb EM and they seem a little too powerful, you can always tone them down by increasing the tension on the return spring or setting the gap on the EOS switches a bit wider.
Lee
