I think Quantum Slipstream in Star Trek is more like a path than a tunnel (e.g. Transwarp). That is, you ride on it rather then through it.
Doesn't make any sense to me but hey, it's Voyager.
Another way I've heard it:
Quantum Slipstream is an area in subspace that is already moving at warp speeds, when you jump on to the area you're automatically at a high warp velocity, and then you can add to it your ships normal high warp velocity so 9 + 9 = 3032 times the speed of light.
I believe Slipstream is faster then Transwarp.