Powertune doesn't just throttle for heat, it also throttles for power consumption (hence its name.) You are aggressively overclocking, so you are surely exceeding the amount of power the card is willing to draw. You have to do some tuning to try to convince it not to throttle, but even then it's a losing battle. Your card is not designed for compute, and is known to be problematic with oclHashcat.
There's absolutely no way your card is only drawing 265W @ 1100 Mhz under load. The 290X doesn't even draw less than 300W at 1050 Mhz. So GPU-Z is absolutely not to be trusted with those measurements. The 295X is a 500W+ card, pure and simple. Your card is not designed for overclocking, and absolutely is not designed for compute, so you're certainly pushing the power limits of that GPU. That's certainly why you are seeing the GPU throttle.
There's absolutely no way your card is only drawing 265W @ 1100 Mhz under load. The 290X doesn't even draw less than 300W at 1050 Mhz. So GPU-Z is absolutely not to be trusted with those measurements. The 295X is a 500W+ card, pure and simple. Your card is not designed for overclocking, and absolutely is not designed for compute, so you're certainly pushing the power limits of that GPU. That's certainly why you are seeing the GPU throttle.