I think there may be some confusion. The in and out is referenced from the computers point of view. So, my Mix Pre 6 has 6 inputs for mics and instruments and then the 2 mix channels (Left and Right). That is how you get to 8 inputs TO the computer. Again on my Mix Pre 6 the 4 out just means that I can get 4 streams OUT of the mac back into the Mix Pre 6. This allows me to add 4 different things from the computer, usually Logic, to mix back in for recording or in my case just for listening back.
This is why I was questioning your need for 8 out. The 8 in, depending on your interface, would give you 8 discrete inputs for your drum mics and then pass along all 8 as ISO's (Isolated Tracks) to Logic and perhaps even a mix as well making it 10 IN. I just didn't see why in your description of your setup why you need 8 channels of audio to come from Logic to your interface.
I hope that clears it up.
BTW, if you're trying to just do 8 drum mics I wouldn't use the mix pre 6 since it'll only give you 6 discrete inputs to the interface itself. If you want super quiet pre-amps and the ability to record the track to an SD card simultaneously with recording in Logic then you may want to look at the Mix Pre 10 line and since for music you may want to look at the Mix Pre 10M. If you don't need that level of quality in the pre-amps and simultaneous recording to the SD card, I'm sure there are other interfaces out there that would give you the 8 ISO's you need for less money. The standalone mixer like you had may be what you need, you'd just need to make sure that it gives you 8 ISO's to the computer not just the left/right mix as the one you mentioned says it does in it's specs.