Re: How fast is quantum slipstream drive?
Just for fun, by my favorite set of TNG warp equations, 260,000c would be around warp factor 9.999997359835618. Be home in time for dinner!
I'm sure that when Starfleet impliments a drive capable of such velocities, they'll either retool the scale (So that said speed is Warp 21 or something), or add another scale for said drive (I.E. Transwarp Factor 1, Transwarp Factor 2 etc etc).
I promise you I will listen to what has been said here, even though I wasn't here.