From what I can remember reading, the throttle position sensor voltage level at WOT should be = but no greater than 4.6VDC. If you have something to read or measure this voltage at obd2 level or right at sensor level you can carefully dremel the throttle stop plate till you read like 4.599 volts. That is the only throttle position sensor mod I can think of, correct me if I am wrong.
4.75 - 4.77 millivolts is the max you grind it if memory serves me correct. If you go over it, you'll throw a DTC.
The first thing I'd like you to check before you start grinding away at the stop is to open up the throttle all the way first. If the blade is perfectly flat when open, don't grind. Otherwise, grinding it down will cause the blade to over open, and act just the same as if it weren't opening enough no matter what voltage signal you get off of it.
The premise of this mod was to get 100% throttle opening out of it by grinding the stop. Generally the PCM reads 100% throttle as a millivolt signal of 4.75 - 4.77 as I mentioned above. So accordingly if you're getting 4.2 millivolts, it may be only opening 92%. But without a visual inspection to go along with this, you can't make a determination as to the accuracy of it's reading. For instance, if for some reason yours does read 4.2 millivolts, but after visual inspection, you notice that the blade is in fact 100% open (I.E. flat), then there is no need to grind no matter what it reads.
Doing this also won't net you any large gains though. It's more of an operational type thing.
Nope, just where it stops. In turn, the more it opens, the higher the voltage. But once to a point, it will trigger a DTC (setting off your check engine light).
Some people simply see that it may be only open 97% (or something like that) and think that it (the throttle blade itself) is only open 97%. Hence grinding it enough to get it to open to 100%. But what you really want is at 100% open, the throttle blade to be perfectly level (flat) regardless of what actual millivolt measurement is being taken.
That is why I would check the actual level first before resorting to grinding as once you grind it down, you can't put it back!