Advertised stall speed ratings of shelf-stock torque converters are simply guidelines, and are generally based on naturally aspirated torque output regardless of the manufacturer. A torque converter will provide a range of stall speed based on the load that it is subjected to. Most shelf-stock converters that feature 3,000+ RPM stall speed ratings and that are built for GM transmissions are usually built from a 245mm core. This is a common core that works very well in a wide variety of applications. However, in order to get higher stall speeds from this converter core such as your 3,800 - 4,200 RPM advertised rating the converter will typically need to be built with a negative impeller fin angle. While this type of build will achive good flash stall characteristics, it also has a negative impact on the efficiency of the converter, and this type of converter design will stall much higher than advertised when used with a combination such as your turbocharged big block and transbrake. This type of converter design is best suited for use with a medium horsepower small block or mild big block in a street/strip car or bracket car. While the converter may be capable of physically withstanding 800 flywheel horsepower without experiencing a failure, that unfortunately does not mean that the converter is going to function properly in terms of stall speed and efficiency with that type of power level. The stall speed that you're observing is not an issue of horsepower but an issue of converter design versus the input torque it is subjected to while on the transbrake. You are not going to find a shelf-stock torque converter that is going to provide the performance characteristics that you desire with your combination, not only in terms of max static stall speed, but also in terms of efficiency at the drag strip. You need a custom built unit if you want to maximize the performance potential of your combination. I offer member discounts if you're interested. Let me know if I can help you out.
Thanks,
Pete