How out-of-flat is the workbench? Bear in mind that the whole workbench does not really need to be flat- only the area around the front vice and to the right of it where you stand.
The #7 is not really the best hand plane for the job if you need to flatten a large area. If you have a scrub plane, low angle jack or any similar plane (preferably with a toothed blade) you'll be done very quickly by removing the high spots without getting knackered. Then finish up with the #7. I actually quite like the grippy surface that a toothed blade leaves.
Unless your top is really off, you'll be done with the hand plane method before you've finished putting the router sled together. I like using my router sled for routing out recesses that the hand planes I own can't reach such as chair seats, but I don't favour it for top-flattening work.