It won't function 110% correctly. IIRC, a few of the gauges may not work properly.
You also wouldn't technically gain anything-- it wouldn't raise the redline/speed, etc.
The biggest things are the 03/04 clusters are interchangeable amongst like models... IE: an 03 GT cluster can go into an 04 GT and vice versa. Same w/FWD and AWD.
The 05+ clusters are different electronically than the 03/04 and won't work in those years. IMHO, the 03 GT has the best cluster with the red gauges. My 04 GT has the white gauges and I loathe it.