My math is a bit wrong I suspect, but I think that means you're saying that after demob the 1000M away squad should be able to have something like an 88% speed difference to the demobbed target.
The problem I see with this is that I think it will discourage fights, because I dont think 53 seconds will be a reasonable amount of time for a hypothetical rescue squad to deploy and get to our hypothetical demobbed guy. I suspect such a mechanic would merely discourage small fights, as you would have to write off someone caught at any distance as a unrecoverable loss.
IMO, demobs should enable one to force commitment to a fight, but not to make it hopless to try to rescue someone thats demobbed.
As such I would propose a mechanic that results in double demob on someone thats doubleplated to result in the doubleplated fellow to be at say -30-50% speed, and not -70% speed.
In my limited experience, I would believe this to be a better balance between forcing fights and just providing gank targets. From the PVP (very little, I admit) I've been in so far, it seems safe to say that 2-3km is a reasonable travel distance. For 2KM, even a lightweight squad averaging 75kph (im not going to say sillyfit to 100kph even though I could see a lolintaktecm squad doing that) will take about a minute and a half to join the fight. So IMO, a time difference of 53 seconds is not enough I think.
As neoxx said, being able to fit passively to mitigate any demob attempt is kinda silly, but neither do I belive they should be something like a 'lolnano' button, for lack of a better descriptive term. Allow demobs to force a fight, but dont make them so powerful that the plated+demobbed guy has no chance to be rescued.
Balance to encourage uncertainty, which I think encourages small group combat, and less blobban.