255/55/19 is huge in Transporter terms, it would result in approx 10% increase in gearing over factory wheel /tyre combinations and a proportionate reduction in braking power. Your speedo would read miles out too. (under read)
Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.
www.willtheyfit.com
In my opinion a 10% increase in gearing is too much, others are of course free to disagree!