Coen, the 12kHz bump is what you get with the 288's - that's normal. The bump goes away off-axis, so the power response is usually quite smooth. I think that the response of your realigned 288's is very acceptable. I suspect that GPA diaphragms won't bring anything much better.

288-8k on Lecleach 450Hz horn
LECLEACH.jpg

same with hf correction
LECCOR.jpg

at 30 deg off-axis
LEC30.jpg