That's the problem. The speed, if shown, would be the speed for the ship you are currently in with the exact loadout you currently have and the cargo you are currently carrying.
It isn't uncommon to engineer your thrusters then either transfer them to a different ship or change the loadout on the ship and find that they don't give the advantage that you hoped for with that loadout.
That is especially true with the enhanced performance thrusters which have a narrow range between minimal and optimal mass. A comparatively small change in ship mass can dramatically change the performance characteristics of the thrusters.
Absolutely the UI is confusing to many players and could be improved. By all means it could show a representative speed for the current loadout. Remember, however, that if players focus on that one number and ignore the scary science they run the risk of being disappointed when launching in their "finished" build.
Whilst it's understandable to want to know the impact of the engineering process on the ship as a whole the engineer is ultimately not modifying your ship but a single module and the UI reflects that. A player who takes the time to understand the principles involved will always have more confidence in the results, which is true in any field of study not just this particular game.
My advice for any engineering is not to replace a modification with another if you can afford to buy a second module and engineer that. Then you can compare the two results in the real world.