What do you mean FOG?
I don't think he knows WHAT he means by that. He keeps saying it, BUT his statement is patently not true.
If you slap an aftermarket pipe on your bike, without doing anything else; Yes there is a good chance you will lose power over part of the rev range.
That is because slapping a pipe on is only part of the job. The pipe changes the tune of the motor. You MUST compensate for it.
If you change ANYTHING major from what the engineers at Kawasaki designed the tune for, you will have to change the tune of the motor.
If you change the cams, to anything but OEM, in your motor, you have to tune for them.
If you change the pistons, to anything but OEM, in your motor, you have to tune for them.
If you change the stroke or bore of the motor, to anything but OEM, in your motor, you have to tune for it.
If you change the carbs on the motor, to anything but OEM, in your motor, you have to tune for them.
If you change the length of the pipe on the motor, to anything but OEM, in your motor, you have to tune for it.
You have to ask yourself why EVERY race vehicle, except for stock classes (which FOG used to run), use an aftermarket or non- stock pipe. EVERY ONE! Do you think FOG knows more than the race engineers at Honda? Yamaha? Kawasaki? Ford? GM? I don't. Not for a second.
The way I see the problem here is that your hero FOG makes these blanket statements, but doesn't explain why he makes them.
If you know about what a pipe does and how it does it, you will find that the stock pipe on a EX500 is a pretty good all round pipe as stock pipes go, and you can even tune the stock pipes a bit by changing their length (the length of the pipe dictates where in the rev band the thing makes power. All things being equal a longer pipe will make power lower in the rev band, where a shorter pipe will make power higher in the rev band) AND the cross over pipe really helps the mid range... but there is the rub... you can't change the mid range tune unless you move that cross over. That is where a two into one pipe comes in. Instead of having a cross over, it has a collector. There is a lot more to pipe than to escort the exhaust gases out the back of your bike, it is an integral part to the tune of your motor.
A pipe is not a silver bullet to make power (there are no silver bullets to make power) BUT there comes a time in tuning where you're going to find that the stock pipe just won't cut the cheese any more and you NEED something else. (NEED is the key word here).
... if I had a stock EX500: I would find LOTS to change BEFORE I NEED to change the pipe (from personal experience I found I had to change the pipe when I had the suspension tuned well enough that I was dragging the stock pipe on most corners so I got a pipe that tucked in better for more cornering clearance, not for any performance/power gain. After I stroked and bored out my EX I had Hindle make me a custom pipe with a tapered inlet and bigger pipes)(No it wasn't worth the effort. That was the last EX motor I built. It was STUPID fast, but a firecracker).
A pipe is a tuning "Tool" and like any other tool, it is useless unless you know HOW to use it. There is lots of good information out there, just not on this site it seems . Find it and use it.
There is more power in most
aftermarket pipes, it is up to you to find it, and knowing where to look certainly helps.
Here is a free tip: a O2 sensor with logging is a great, inexpensive tuning tool.
Here is another EX500 tip: Don't touch the motor till you have sorted out the suspension/handling of your bike. A stock EX doesn't NEED any more power.