Hey,
Ball bearing cores do not affect a turbo's flow capabilities. If you went twin T25's on a pissy little 1.8L motor I reckon you would have to go ball-bearing. And even then I'm not sure it would be able to spool them up. Although, I've never seen it tried so I could be wrong.

What turbo you choose will depend on how you want the motor to behave. A smaller T25 will come on boost earlier but won't make as much power up top as a larger T28.
A bb T25 will make the same power as a normal T25, but will spool up quicker again. That is generally speaking of course.

And with the twin T25's, I think you'll find you will get bucket-loads of flow down low, but because they are still T25's they still won't be able to hold it up top.

I would try and find a BB T28 off a S15, GTI-R...etc. They're cheap and ( I think ) will bolt straight up without hassle.

Anyway thats MY thoughts on it...