Years ago, I bout a set of 12 holes from Cobra Marty. I haven't seen him on web board since about that time. He does have a website up for brake mods.
I did install those in a '98 XJ. They ran really great when initially installed. Then a little less and a little less. Also, I had to replace the spark plugs. Gaps were way to big.
I also installed some in my '00 XJ. I was doing some other work, so I didn't get it on the road for a while. They seem to run OK. Still, I can't say there is any power increase, or MPG. I don't know what stock ones would do in that motor.
I have installed the Ford 4 holes in two '98 XJs. No real power increase nor MPG. Still, the motors seemed to smoother.
I also ordered the second set off of E-Bay.