It sounds to me like you are way past your peak torque and the converter is dragging you down because of it. But I don't race a lot and there isn't a whole lot of information about your motor build, so I'll assume have a crate motor since you have assumptions about your power level. Probably a little off topic but in my experience crates rarely match advertised power levels for a variety of reasons (you don't use all the same top end they do, variances from build to build at the machine shop, etc...). Anyway, since you have a smaller converter it is only costing you the time to install it to confirm the theory.
There are also other possibilities though since you are shaking down a new combo. I've had a couple weird fueling problems in the past...I used to run out of gas at the top of 2rd gear going into 3rd and die (stop accelerating). I had to literally lift and then hammer it again after the fuel pump caught up. Is your fuel pump up to snuff? I changed the line out and then found a new problem when I was running 2 weeks...once the car is HOT...I literally was boiling the fuel in the line. That was a simple fix to move the lines off the block but it translated into the car basically dieing on me (no longer accelerating) just as I shifted it into 3rd again.