05 rx8- engine feels flat below 6500rpm
 

I have a 05 rx8. i just had the motor replaced 1 month ago and the car has had a lot of misfire issues ever since. i took it back to the dealer and they fixed the miss fire with new coils and fuels system clean. now the car doesnt mis fire but it just feels dead below 6500 rpm. above that mark the car is powerful and i can feel the transition when i run it up. is this normal or is there something wrong? any ideas? thanks for all the help.