GTR 32 - I installed aftermarket gauges whilst still retaining the oem ones, here's some observations I've made:
Std water temp gauge => aftermarket gauge can move 30 degrees (in 2 degree increments) without the oem one moving at all (only time my stock one went to hot was "after" I split a radiator hose - this is also common with Subarus).
Std oil pressure gauge => accurate up to about 4.5 bar (agrees with aftermarket one) but won't read any higher pressure, unlike the aftermarket one.
Oil temp gauge => no oem fitted, so can't comment.
My aftermarket gauges are electric Omori ones & have very quick response times (particularly oil pressure where you get rapid changes).