After much contemplating I decided to go with the 20mm front torsion bars and see what happens.
If I want further improvement after that I can still opt for 27 or 28 mm rear bars.
The roads here are really really, well, not good and we also have the infamous Belgian cobblestones to take into account.
If anybody has any use for hollow 21mm Sway-A-Way torsion bars (10k miles), please PM me!