Hi Ali,
I run one for a few weeks in my built 993TT with k24/18g turbos. (Cool stuff but "lighter" vs our EVC6)
This EBC let the turbos work on the wg until the "START BOOST" value, on my car wg are set 0,45(oem) and SB set at 0,8.
I noticed the later you set the SB the better is the spool.
"SET" is the boost target from the WG basis, you lower or raise the boost with it (mine is 60% on low boost and 72% on high boost).
GAIN is raising the boost target (SET) from the spool to the redline, 6% is almost flat. You can use this function to help too small turbos to keep boost at top end for example, or to map a kind of ramp up after the torque.
You speak about a 21,5 psi "spike" from 18,5:
-if the spike is before regular 18,5 a lower START BOOST value will avoid it.
-if the spike is at top end a lower GAIN will avoid it.

Cheers,
P.

Hmm, you can't compare like that, the "SET" value you need is up to your wg pressure basis and boost target.
For example a 45% SET value with a 17psi WG will perhaps give 18,5PSI target boost, with a 10PSI WG you should need something like 65% to reach 18,5PSI.
Adjust your SET value to achieve your boost target with your setup.

(Anyway my 997.1 is not fitted with a Greedy)