If you explode the re-imported (reloaded) component in the original but otherwise useless model, it will work as if you'd modelled everything in place there.
Your imported component will still be in the model file though but you can easily get rid of it by purging.
I do this (in a much smaller scale however - city block vs. individual buildings) but have done it successfully with a whole (Medieval) town and its churches. You can see some shots here. The extent of the model is about 1.3 kilometres / 0.8 miles so there was no such problem however I had performance issues.
As for modelling and geolocating, it can be used for GE quite efficiently although you always need to keep in mind that GE is a spherical projection and SU is orthographical so after a while this will cause conflicts.
Try it out with a simple, not crucial model first.