Maybe this applies for borg travel over big big distances... like how you can't do warp in a solarsystem... (high warp I assume) then maybe to cross vast distances the borg use a 'faster' speed... BUT - it only works in sparsely populated regions of space - with few stellar occurances - a prime case for this would be the 'voids' between arms... ok there might be a few stars - but nothing like in the arms... so once the borg cube reached more stellar dense areas - they can't do the 'super speed' - maybe this is something that stops them from attacking the federation en masse - they have to pass through a few ARMS where they are restricted to speeds like Federation starships. once they get to the 'voids' they can go as fast as they want - but once they get back to the arms - its back down to the ole warp speed - sorta like galactic speed bumps. So from J25 they probably had to cross only one void part - and boom they could have ended up at Jouret - on the edges of Fed space - which also could be restricted by one of these sparse star areas...

I don't know if this is the same was the thing from 'night' in Voyager - since you'd be able to see stars - they'd just be further away - the Voyager thing from night look like swirling light absorbing gas - like a pitch black nebula...

They didn't SPECIFICALLY say in 'night' that it was the 'void' between the galactic arms.

Andrew

And if their super-duper fast engine only worked in huge voids, it might concievably be faster to go AROUND the galaxy than through it.