... The track I use is very soft so the ~3% difference in drive speeds between the front and rear wheels shouldn´t cause any probs right?
The different tyre sizing between front and rear will accentuate "transmission bind-up/wind-up" (making it harder to exit 4WD-mode without simultaneously performing multiple backwards-forwards gear-changing movements) but this'll occur only when driving in 4-wheel-drive on a solid surface like concrete or asphalt (which is never recommended anyway).
In my opinion the different sizing will have no adverse effect when driving off-road in sand, loose gravel, mud and suchlike.