Well, I haven't done any analysis on this as yet so I can't say for sure what difference it would make. But whether you're sucking, or pushing it's pretty much the same thing. Normally you would have a 1Bar differential, with a turbo you might have 2-3Bar differential. And we all know that more static pressure does not equal more flow.
As for boring out the standard ones to 48mm - Well you would then need to find a spindle that was slightly longer, a 48mm butterfly, someone who could bore it out reliably so the spindle bore was still in the right place. Oh and then your inlet manifold and plenum wouldn't line up because they are still 45mm... but apart from that it would probably be easier
But seriously, you just can't bore out throttles to bigger sizes.
Slide throttles are notoriously difficult to get to seal and to move when under pressure, and can stick alot. Roller barrel throttles are great at full throttle, but rubbish at everything else (similar for sliders). There is a reason why everyone uses butterflies. These are just butterflies, but a little better