Wide open throttle =WOT
Yes, cut the header instead of the pipe. Very easy to do with a pipe cutter, A hack saw also works well just be sure to clean any chips and burrs prior to assembly. It is often a lot cheeper to replace the header if you cut it too short. As you mentioned you have a carbon pipe this is very true, even more so. Also you want to start the divergent cone as soon as is practical.
With most after market pipe set ups you will want to maintain a gap (2 mm to 3 mm) so that the header and pipe don't rub causing particles that could foul, damage your piston and cylinder fit. You want to keep this gap small so as not to allow the coupler to expand like a balloon or be exposed to the exhaust heat and burn.

As Bax said you can't "trombone" the pipe. It doesn't work. There is too much stuff going on. A pipe is a very simple yet dynamic apparatus! For one thing you will need to adjust the mixture strength as the flow dynamics change. You really can't do this while tromboning the pipe.