Thanks for the links, especially for that one on how to calculate mufflers and pipes.
Regarding the removed middle muffler, Supersprint
is producing one similar. Just do not know the diameter of the middle pipe.
Think I will start with the exhaust manifold (headers) first, since the win should be the greatest there.
Then continue downwards.
Bear in mind that I need to make it such that it satisfies bloody TÜV (MOT)
will think about making easy replaceable middle section (using V-groove bands), to be easy to swap before TÜV (MOT) check
for the rear pipework, will seriously think about cutting off the "dish" for the spare wheel and make space for something better
EDIT: and yes, if middle muffler removed, think that 2 pipe routing would be possible with 3 in pipes! "meditate on this I will ..."