The mah has nothing to do with output voltage, that's the capacity of the battery to hold a charge, 1000mah will hold a charge longer than 650mah.
For an Ego battery get the 2.0 ohms. 3.0 is kind of high for that output voltage, it'll work but will be a weaker
vape in comparison to 2.0.
The Ego is regulated so it should be consistent throughout the charge
Burnt taste or dry hits can be caused by several things including to much power, not likely with a Ego. Wicking issues which would be dependent on what your using with it, some have more issues with wicking than others or running it dry, no matter what you choose to use make sure it's wet so it can feed the coil.
Boge are tried and true cartos, you shouldn't have any issues as long as you keep them wet.