sorry to sound really stupid but what do these do?

 

It alters how high or low the fuel cut is.
e.g. WRX (1995/96) ECU Z4 - 12.71psi, dropping to 11.78psi @ 6600rpm. Fuel-cut 15.65psi

With a fcd you can raise it above the 15.65psi therefore raising the boost level with a boost controller

 

Alternatively you could get a remap on a decent ECU, i.e. an Ecutek. This will allow you to raise fuelling pressures and boost levels but still ensures your engine is protected against silly boost levels, knock etc.

Basically, if you get a fuel cut defender and a boost controller you can just wind the boost up until the wastegate stays permanently shut. Your car will go pretty well.........until it goes pop.

Also, getting a re-mappable ECU allows you to remap the engine for better performance when you've added other performance parts on, i.e. exhaust, stroker kit, up-pipe etc.