Charles,
The difference on the roll out of the slicks will move yours on up to 18% slippage. The slippage isn't always about how old the converter is, but how efficient it is. Age can make them inefficient, but they can be built that way also.
I feel that a good number is 300-400 rpms of slippage and mine is at about 450rpms, so I would definitely be looking at any converter that has over 10% and much over 500rpms of slippage. This is horsepower lost that is not moving the car.
If you were able to have your converter rebuilt to where it only had about 10% slippage, which would be about 500rpms then that would put you at about 7000 at the stripe. What rpms' does your engine fall to on the shift as this is the effective stall of your converter? Mine drops to about 5800rpms on the shift into high gear.
Bill