The cage is bolted in, but they have welded strong points in the front footwell next to the sill. And at the bottom near the B piller. I rekon these could be cut out if you needed them to. As far as trim goes you could cut small holes in the side panels in the area where the rear arches are, and fit that trim back in. Those two pannels are pretty big and are all once piece extending from the boot all the way to the B pillar. The same goes for the plastic trim that sits over the side sills. However I have opted to just keep them out of the car, and uncut.
They say the cage weighs in at around 40 kg or so. it is pretty hefty. With trim and rear seats out it will compensate to an extent, but I certainly have not noticed the difference in performance. It does also stiffen the chassis up noticeably, so that is a good thing.
Oh and the sun visors are removed, though I guess you could have them in, but in the down position. I will just fix a sunstrip, probably along the lines of: 'On a mission' or something...
I will add that these guys will pretty much be able to build the cage to your needs, from a simple 4 point to the full works, gusseted(sp) the lot.