My other car is a MINI!!!!
in reality, the 220+ bhp figure you speak of is typical for 20-22psi of boost from a ko3S.
At 15 psi, yours will sadly be more likely around 200ish, and not running what you believe it is.
There is almost certainly scope for more power if you're at 15 psi currently, and the standard AUM fuel system is more capable than the turbo so that's not something to worry about.
Chances are something has happened since it was mapped causing the loss in boost, and consequently power.