You have Ecutek, just open it up in the app and see what your DTCs are. Then we can help.
FWIW, all of the "heavily modified" cars get drivetrain errors. All the time. Then we scan and fix. It's very likely not transmission related, since you're likely in limp mode.
Whatever you say boss. You're right, there are a lot of 850-900hp stock block builds, there are also a lot of 'the bitch kicked a rod at 700hp' builds. Especially with running nitrous. Be careful you don't run it too hot.