OK, well these settings are in fact correct for the Miata as well as the Porsche if you want the power output to be relative by way of telemetry. This is because you would be using the SAME .299 specific output (25.1 / 84) for each car. So the difference in power and feel between the cars is completely based on the telemetry being output by the car through iRacing.
BUT Yes the Miata feels really weak at those levels as the car itself only will do about 15Nm (excluding crashes) in telemetry which would mean the wheel in normal running would only get to about 4.485Nm (15 * 2.99) of your TD set 25.1Nm under heavy cornering.
That is honestly one car that I adjust up from with my settings, and the one car I drop down on is the Indy Cars.
If you decide to let iRacing handle the relative power differences for the cars everything pretty much becomes “Set it and Forget it”
Though the best way for you to alter at this point is to just lower the Max Force Number if you want more strength at the wheel (raising the specific output) and if you need less force at the wheel then raise the number (lowering the specific output). This can be gone on the fly through the racing black box as well which makes things quite nice as if you start a long race and then find in the middle you really can’t hack the power you set you can then lower it while you drive… Or visa versa if the car feels vague and lifeless you can raise the power as well.