8 motor base

How effective would an 8 motor geared X drive do

It depends on virtually unlimited factors. How are you using the rest of your motors? What’s the theme of your bot? Any catapult? Lift? Intake? Hang?
In general, it’s debatable. Check out GER for an example:

It would have a bunch more torque than a 4 motor X drive that’s for sure.

Can you get away with having 2-4 other motors for the rest of your robot? If so, go for it!