For 5mm, then you can keep the studs youve got. 10mm is getting borderline, anything more than that you want longer studs.
Dont be put off by the scaremongering, lots of track and drift cars run spacers without any problems, in fact Im surprised at the thread on the GTR board as Ive never known any fail at all, the 5mm ones simply cant fail due to their design anyway.
Ive been running 25mm spacers all round on my Z33 for 3? years now, fast road, track and drift use and its never even once entered my head that there could be a problem due to them, I know guys running 35-50mm without issues too