I have travelled this route many times and it is possible to take the VFR, but I can assure you,you would not enjoy the experience, and your wife would be unlikely to thank you.The Molesworth road is controlled by the Dept. of Conservation, and open to the public for only 1 month of the year (Jan. I think ). The rest of the year it is used only by the large sheep and cattle stations in the valley, and maintained accordingly. After heavy rains there are washouts and mud, and during the dry,lots of loose gravel and dust from all the 4x4's converging in the area during the 'open month'. A toll is also payable,($15/bike I think).
I have always found this route quite boring, it basiclly just follows one long valley with scree slops on either side.
Why not use the Inland Kaikoura route turning off just south of Kaikoura? This road is nearly all sealed and very scenic.Another alterative would be the Rainbow road from St.Arnaud-Hamner Springs, much more scenic, better road surface but still prone to deterioration after heavy rain.
The dillema you face with tyre pressure on the VFR is that the more pressure you run (to avoid fracture punctures) the less traction on loose gravel you have, due to the profile of the tyre.
Which ever route you choose,take it slowly and enjoy!