Regarding the difficulty on driving in all the GP4 levels, You can find in each magic file for each track the strings You above reported. But, note, please:
the "power" factor is the percentage of car power bhp for ALL drivers ( including therefore human one ) for THAT track ( assuming that in GP4 100% of car power is 500 ).
Therefore, You can increae this factor to make lower the lap times for all drivers in that track, but this could mean that also difficulty on driving against CPU drivers is higher for Human driver.
The string in magic file with "grip"for each driver level is the ability of ONLY CPU DRIVERS to retard braking before the corners, and the grip during the corners. Therefore, increasing the value of this string, You can increase a lot the difficulty of performances. If Human driver wants to have less difficulty on driving against CPU drivers, the value should be made lower.
In practical, the more You increase "grip factor" in magic data, the more Human driver will have difficulty, while increasing "power factor" both human and CPU driver will have car more powerful on that track.
Finally, remember that GP4 default magic 100% value is 500 for both strings, but while commonly "grip power" can be also higher than 100 % ( 500 ) in some tracks for Ace level, "power factor" is always maximum 500 for Ace level, and lower for the other driving levels.
And, don't confuse "grip factor" with "track grip"...they are different parameters.
Hoping to have clarified these aspects.