Sounds like you've covered about everything.. Do you have a under-drive pulley? If so and you haven't changed to the matching water pump pulley do so it will help. The only time I've had the problem you have now is when I still had air in the system.. Funny, at 2500 rpms .. your turbos (if stock) start working at around 2600/// And if everything is as you say I would be cleaning out the fins too on the radaitor.. Maybe check the fan clutch