Yep, that's definitely the cause of your problem. Your G40 is making all this boost, but as the flap isn't closing, the boost isn't going into the engine. This not closing will cause a massive loss in power.
The next thing you need to check is when you put your foot to the floor (with the engine off - don't want to go bouncing off the limiter), check that there black curved bit that the throttle cable cable goes through won't close any more by hand. It may have a small bit of movement, but that's fine. But if you can move it to the point that it causes the flap to close, then either the cable is out of adjustment or something is preventing the accelerator pedal from going to the floor. Check that nothing is getting in the way of that.
If the cable needs adjusting, just before the cable goes around the black bit, you will see a ribbed plastic cover over the cable as it passes through a small channel. There should be a little clip on this. Make sure it's actually there and adjust if needed.
If there's no extra movement in the little black bit, check there's nothing obstructing the movement of the little arm that connects to the little flap.
If there's nothing blocking it, someone must have played around with the linkage (that's the slightly bent metal bit with the beige ball joint sockets). Adjust this accordingly on the threaded rod.
Please also note: It is quite normal for the boost return valve (the flap) not to start closing until the accelerator pedal starts passing that half way point.
As for the cause, it depends of which of the above is causing this...
Either way, this should be a very simple and cheap fix.
