-- Last Updated: Mar-07-13 11:24 PM EST --
My Mini is old, a '94. Back then, as you can see in this pic, the cross bars mount through the roof skin directly to the frame inside. It's an incredibly strong connection, but the posts have to be in the positions to match the threaded holes inside:
The cross bar spacing is short, which is why I used the EZ-Vee carriers to place the boat supports far apart. This type of carrier is very secure, and the boats never move.
As of 2007 (I think it was) the new cross bars hook into a hidden rail just above the door line, the same as Audis and VWs. The new setup look to have about the same spacing, more or less, although the crossbars look to be a few inches longer.
PS: no to the boat inside, but I do carry one-piece GPs without much trouble. Top speed is unaffected by the boats, but it's a short car and mine has a performance suspension, so it's not a cushy ride. In fact with the long roof up top, it's easy to forget they're up there as you can't see the bow normally. There's little impact on MPG.