What is a Boost Spike
 

As above, can someone please give me a brief definition..

Cheers
Sam

I have seen it mentioned whenever breathing mod (exhaust etc) are done to a MY01 and newer car. Has got something to do with the standard ECU not being able to compute the extra air getting into the engine and thus dropping the boost back down. After I re read what I typed I basically cant explain the whole thing, although it makes sense to me, if that makes sense.
Someone elso will have to have a go
If I find the URL I will post it.

A boost spike is where boost pressure goes for a short period well above the intended limit.

E.g. the car goes to say 1.7 bar during spool up, before fairly quickly dropping back down to the target boost of say 1.25 bar.

Not normally an issue on MY01 and newer WRX's with breathing mods, can be with STi's.