Here is some info on the Z4 ECU:
Boost Targets & Fuel-Cut Limits
WRX/STi Saloon ECU's 2C, U8, W6, W9 - As above, 11.47psi held to redline. Fuel-cut 14.57psi
WRX (1995/96) ECU Z4 - 12.71psi, dropping to 11.78psi @ 6600rpm. Fuel-cut 15.65psi.
UK/Aus/Euro ECU's V7, 7D and 6D - 11.47psi, dropping to 9.47psi at 6600rpm. With Fuel-cut @ 14.87psi
UK Prodive ECU - 13.33psi (peaks to 13.64 @ 5600rpm, drops to 12.71psi @ 6600rpm). Fuel-Cut 15.65psi.
STi RA (1996) ECU 7K - 13.17psi (peaks to 13.64psi @ 5800rpm). Fuel-Cut 15.96psi.
WRX Wagon ECU V9 - 10.55psi dropping to 6.84psi (starts to drop @ 5000rpm). Fuel-Cut 13.79psi
Z4 and UK Prodrive map are near identicle, with the Prodrive Map running slightly more boost.
IMHO the Z4 ECU is the best stock WRX ECU you can get, as it has much tidier fuel and igntion maps. Comparing it to the earlier ECU's I'm suprised the earlier cars run at all.....
A Z4 ECU will also run fine in a UK and give near UK PPP performance
cheers
Wee Jock