What "redline" are you referring to?
The file engine.ini contains a section called [ENGINE_DATA]. Within that is a LIMITER value - this is the RPM at which the rev limiter activates, if there is one.
That same file engine.ini might also have a [DAMAGE] section. This contains a line RPM_THRESHOLD= which is the RPM at which engine damage starts to occur (so it's basically the functional redline). Presumably the LIMITER value is already less than this, but it doesn't have to be.
The upshift point indicated in ai.ini needs to be lower than the rev limiter and the damage threshold. If the upshift point is too high then you'll see the AI revving against the limiter but never engaging a gear.
It's a ridiculously common bug in car mods.
Similar shift points should also be set in "drivetrain.ini" in the [AUTO_SHIFTER] section, if it exists. This controls auto shifting for the player, but if it's missing then the values from "ai.ini" will be used.
On most modern cars, you can just set the AI upshift point slightly below the limiter and it will be fine.
Some cars (mostly older street cars) might have better acceleration by shifting earlier. Adjust the AI shift values accordingly if this is causing the AI to be slow.
If it seems like your changes aren't having any effect, make sure there isn't still a "data.acd" file present. That file overrides everything in the data/ directory. You can either delete the "data.acd", or else use "Pack Data" in Content Manager, which will replace the data.acd with your edits.