JB mushy? listen to rust in peace. rhythym and lead all JB for the whole album. all bridge too AFAIK from the tone.
Rust in Peace was 30+ years ago and is not a modern metal sound. The JB was used because it was then a popular pickup that was common, like the EMG 81 or DiMarzio Super Distortion.
Yes, mushy. They probably rolled off the lows in post with a hi pass filter to some degree.
Fairly sure they also used boosted Marshalls, which are not so much a modern amp for extreme metal vs. Mesa, Peavey, Diezel, Engl, Bogner, etc.
Also I've found the JB mushy in my personal experience. I go for EMG 81s in 24 volts, Blackouts, Full Shreds, and Parallel Axis pickups instead of the JB.
Modern metal is also 16ths at 200+ bpm, sometimes to 240, far in excess of 80s-early 90s thrash metal of the period.
The JB is a fine rock pickup at moderate tempos in the bridge, but I have other things I would use first, even the Duncan Custom line.
You can make up pickup deficiencies later in your signal chain. I happen to like the EMG HZs that many people consider anemic. But I compensate by adjusting the preamp as necessary.