Thanks Turbo Dad!
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I'll try to contact him and see what he says.
In the meantime I've done some minor modifications to the non-inverted setup. I've moved the gas pedal a little closer to the brake, and elevated the brake again using washers. (the nixim mod is of course still installed) I haven't used it enough yet to make a final verdict, but my first impression is that heel&toe is much easier now, because when I depress the brake my foot is almost precisely level with the gas pedal, so I just have to twist my ankle slightly to heel&toe.
Here's a few pictures:
The block I used to move the gas pedal:
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Seen from the front:
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Seen from above:
I'll keep it this way until my Rinoseat arrives, and then I'll decide on whether I want to invert the pedals or not.