AFAIK, the sockets and wiring is already done & fitted for T5 bulbs. its like your signal bulbs, in my Satria for example, its wired for T10. so if I want to use a bigger brighter bulb, I have to replace the socket to do that.
for dashboard wiring / socket must also make sure the new socket can fit. again, my Satria uses t10 for the illumination (speedo, fuel, temp) and T5 for the aircond panel & everything else :)