I do agree with what
Dipstik-sportech said. I've had this issue when shifting from 3rd-4th when my car was stock. With my turbo setup I have the issue between 2nd-3rd, 3rd-4th, & 4th-5th at a high RPM. I don't believe it is clutch related or shifter related (have a Kartboy Shifter). I believe it has to do with the amount of torque at the high RPM and the syncros. Possibly too much inertia for the syncros when applying the next gear. After all it is a new design with a triple cone syncro application.
Now the wait begins until the transmission gives out lol... oh well, the cost of performance, right? Or just bad (and new) design...