apexi n1 is similar and kakimoto both under axle and same style of back box,and angle they mount at.
Wait, you want to know if there is another company that makes the EXACT same thing as the HKS silent power? Why don't you just buy the silent power?
You can't have the exact same thing at a lower price, replica's are never perfect, usually much lower quality...
If you want quiet here are a few:
Apexi NOIR
Apexi Worldsport
Greddy SP
RS*R Exmag