I wouldn't trust a resistance check to confirm glow plug operation.
I replaced all four of mine recently. I bench tested them with a multimeter and a set of jump leads.
1 worked out of the 4
3 had low resistance values, with no discernable difference.
1 had high resistance - yes, it was one of the 3 that had failed
So, in short, I wouldn't say that the test results were in any way dependable.
In any other 'sluggish to rev' situation, i would suggest the lift pump failing, but since there's black smoke there's too much fuel in the cylinders anyway. I'd fully check the air inlet route from filter box to manifold, as well as querying the health of the injectors.