If I run SP mics through a battery box and into the UA-5 w/ phantom power off, am I getting any benefit from the wmod?
Yes, the benefits of the upgraded preamp and the ADC.
The battery box brings the signal up to line level correct?
Depends on which box you have. The standard battery box provides power to the mics and does not provide gain to bring the signal up to line level. A hot sound source
may be able to drive a line level without the added gain, though likely not high enough to get strong levels up near 0dB.
Same thing that the preamps in UA-5 do?
Only if the battery box also provides gain - some battery boxes do, some don't. Regardless, though I haven't done any direct testing, the UA5's gain stage should sound significantly better than the battery box's gain stage - especially with the wmod.
So what happens when the signal is already at line level.
If the signal is already at line level, you'd just keep the gain on the UA5 turned as low as it will go, or feed the signal through the RCA inputs on the back of the unit.