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Initially I was unsure it was what I was really after - since it comes as a 3.5" bay installation piece - but I realised the cables can just be unscrewed from that and used to connect to the top USB headers directly instead of having to chase them out the back of the case.
So with this, the choice ends up yours - either just route the two built-in with the cable from this bay to the mobo, and then use the other 2 on the back, or I guess it might be possible to still route the built-in panel out to the back ones if desired, and use the new bay to connect to the header and have 4 front-facing USB 3.0 ports.