flash convertor?

Aftermarket converters designed with higher stalls than stock have more efficent fines inside the converter for fluid transfer into the tranny. This makes them more efficent at higher speeds. However, the higher speeds needed to get the full bennifit of it's increase efficeny (sp) is pretty high and because it is designed to apply max. amount of fluid transfer at the big end of the 1/4.

Unless you plan to travel at high RPM's, a stock converter will do for the Hwy.
A light stall converter for a mild engine will get you off the line faster, but may have you suffer at cruise. Exceeding cruise (Lets just say 55 mph) to get into the converters most effective power transfer area (Again, lets just say 75 mph) is where the bennifit becomes (Possibly) apparent.

Engine combo dependent;
A converter with 2000 stall may flash @ 2700
The flash point is allways higher than the rated stall.
Consider the flash point a point where the converter has full power engaugement. The point where the engine and trans are working as close as one unit as possible. No slippage.
The best race converters exceed a 90% rate of non-slippage. Meaning the engine makes one full revolution and the tranny is making 90% of that revolution.
(This is also why manual trans cars can use a deeper rear gear and if you can shift the manual trans like no bodys bizzness, be faster than a auto. It was that way for years in the earlie days when converters were not so effcient and shifting was slow. Times have changed and it's hard, if not really impossible to find a person that can shift a manual better than a preped auto)