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've eaten breakfast cereals tougher than you! For reference, they were the ones with little marshmellows in them."