I calculate that my converter has 6.78% slippage. It's an eight inch and stalls at 5200. So is 6.78 normal for an 8 inch or should it be closer to 3-5.
If the slippage could be reduced from the current number to say 4% how much would that affect car performance?

Thats not bad... 8" conv tend to have a bit more
slippage... as to the difference in performance.. I
doubt you would see much at all... besides a lower rpm
otherwise if it was still pulling good your rpm would
be higher than it is... my conv has 7% slippage..
there are some that have less.. about 5% but from
what I've seen or owned its only a few