As I understand it, an EBC ensures the wastegate stays closed until the preset boost levels is reached (this makes boost build faster), then it signals the wastegate to open to bleed off excess exhaust gas once the max boost setting has been reached.
Without an EBC, the engine relies on the wastegate boost spring to set the max boost level, however, the wastegate starts to open earlier/before the max boost level is reached, thus it takes slightly longer for boost to rise/build to the max setting.
Again, that's my understanding. Maybe some of the other turbo guru's here can elaborate more
'89 911 Turbo Cab
Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition