I don't know about locking the ECU but it could void your warranty if you hack the ECU, depending on your dealer and if Triumph central gets wind of it, which they could because they sometimes get involved with diagnosing problems.
It's already derestricted with the secondaries open 100% except at the high end in 4th and 5th above 5000 rpm. For street racing, you shouldn't need more than 2nd or 3rd. There's no point if you're keeping the exhaust stock. You could optimize the fuel for the completely stock configuration with a Power Commander and a good dyno tuner. That has nothing to do with the ECU and shouldn't get you in trouble with Triumph. If your dealer says there's a problem with that, you could remove it whenever you bring it in and reinstall it afterward, easy peasy.