It''s tricky to make a direct conversion between Star Trek style warp velocities and 40k style warp apparent velocities, thanks to the fact that the Empyrean is technically a different universe; so much depends upon "local" conditions and the skill of the Navigator. As a result, Star Trek travel times are far more consistent (and hence easier to put an actual velocity to).
What I can do, is provide the Okuda formula for ascertaining FTL velocity using warp drive (as defined for use in TNG and later), and a link to the original travel time calculation table for 40k Warp travel.
The Okuda formula works cleanly from warp 1.0 to 9.0, at which point there is an asymptotic increase in the exponent of w, meaning it takes far more energy to increase velocity further, with warp 10.0 theoretically allowing the vessel to exist in every point in the universe simultaneously, but requiring an infinite (or greater) supply of power to achieve.
It is as follows:
where v is the velocity, w is the warp factor and c is the speed of light in a vacuum (3x10^8m/s, for reference). From there it''s a case of simply plugging in the distance travelled to find flight time to compare.
The table for 40k travel times appears here (roughly the 15th heading down), and is taken from WD139-WD140 (I''m fairly certain it also appeared in the original Rogue Trader book- W40k 1st edition, back in 1987, but verifying that''d mean a search through the archives or pirating an e-copy to check, neither of which I want to do at this point). Using that table, it is possible to derive a formula to work out apparent FTL velocity, but doing so is beyond my current capacity (it''s after 02:30 in the AM, and I''ve had a long day and a reasonable amount to drink), but even without that, it gives you a very rough guide as to the relative FTL capabilities of 40k and Star Trek.