RR, you're absolutely right about the convoy behavior.
I've been playing Webster's GFO from day 1 of it's release and I've never experienced odd sonar behavior. His solution on the jackrabbit problem works very well and has two side-effects that I've noticed.
1. The escorts stop their engines very often and use their momentum to maintain formation with the convoy and compensate the new ship dynamics. That means that
our hydrophones cannot hear them constantly and
their hydrophones are more sensitive with their engines stopped. That's absolutely normal. That makes them more dangerous and more difficult to target.
2. The convoy ships also change their speed to maintain formation with the convoy leader. That also means no guaranteed hits from 8500 yards and no engine sniper shots from long range.
In my opinion, Webster's fix makes the game more interesting and unpredictable as RR pointed out. I've no experience with JRex's SAP mod so I cannot comment on that.
One thing is for sure. FOTRS desperately needs a ship maneuvering fix. The ships behave exactly like stock.