I saw that 2x overdrive puts out about 1.5x light but what does the 4x overdrive do for lumens output?
Is it possible to do a 8x overdrive?
There's no calculation, only an estimate. And it will be different for every tube and ballast. Expect around double the output, but might be only 1.5 times or might be nearly three times.
You can overdrive as high as you like, bulb lifetime will decrease, electricity consumption will increase, and fire hazard will increase the higher you go. Eventually, the tube will blow almost instantly, the ballast may fail to start, or the ballast may be damaged (unlikely).
Remember, overdriving isn't magic and it isn't something for nothing. It is a means of producing higher light intensity from standard fluorescent equipment and the price is your utility bill goes through the roof. There are other means of producing high light intensities that are more efficient and safer, check them out before you decide to overdrive.