I don't know about converter slip % but, my cheep 3000 stall converter, when through the traps at 6000 rpm.
My high dollar 4200 stall converter, when through the traps at 5700 rpm. and picked up almost 2 mph nothing changed but the converter!
With 4.30s and 28" tires, the calculated 6000rpm is 116mph, which is what your tach would/should show if you had a clutch, which would be zero% slip.
If your 3000TC trapped at 108, that would be 108/116=.931 or ~6.9% slip.
If your 4200TC gained 2mph, say 110, that would be 110/116 =.948, or 5.2%
Fun with math
EDIT
If your powerpeak is at 5400, then 6000 is somewhat off the pipe; and 5700 is more like it. The 4200 is also converting less power into heat, leaving more power to gain speed, which should translate to a slightly quicker ET.
