I changed all of my bulbs from the Sylvania LL194 (2.75 watts) to the LL168 which is the same style but about 3.5 watts. It actually helps quite a bit. The tach bulb is an oddity though...good luck finding a brighter one that fits that unique socket. Check out Holden at
Vintage car parts Classic car parts Lucas car parts & Belstaff Jackets from Holden Vintage & Classic in the UK. They apparently used to stock part number LLB501 for a 5 watt bulb. It replaces the Lucas 2 watt original bulb (LLB297). I never tried them because it wasn't worth the postage to the States. Last, check the voltage at your gauges. Many dimmer switches/boards are known to drop voltage and you might only be getting ~10v at the dimmer.