Buschur has one of the lowest reading mustang dynos around. Other dynos have been producing more. Though it is only the difference between stock and the modded part at question that will matter.
Anyways, evos used to make about 235 awhp with 286crank hp,215awhp IS low for a 300 crank hp car
As the motor breaks in it will make slightly more power.....why does 30% seem high....isn't 18% used for rwd manual and 23% for automatic rwd. So you have twice as many moving parts 30% doesn't seem out of the ordinary.
The numbers seem low because Buschur's Dyno is a Mustang dyno. This is not a dynojet.