Don't know too much about the scene in the UK, so I can't comment on "rotary specialist"...
I do have experience with the DynaPack dyno...
My personal car that I did some quick tuning after swapping turbos - the old turbo died.
1987 FC turbo, 13BT, compressor upgrade T04E "60 trim", 10psi
Not as much power as yours, but you see the general trend of the graph...
Thick red was baseline run
Thick green was after tuning the Haltech E8 to get rid of that dip from 4.5k+.
That dip sorta kinda looks like yours...
So I know you can tune it better to get rid of it.
The above dyno was only about 30 minutes of tuning.
It picked up 10 - 15 lb-ft of torque from 5k+.
I maybe did 3 runs total after baseline.
I was surprised it managed to hit 240 on the DynaPack.
Adjusted for DynoJet #'s, conservatively, it's in the 260's range.
The car was still running *stock* fuel pump, so there was no reason to go any further.
-Ted