Do you think this water circulation is adequate?

You should just have them in "directions". Try to have the water streams from the pwerheads cross each other. It creates a chaotic, random flow, which is much better than a constant, steady stream, IMHO

 

I agree

 

I have heard that some reef keepers use 4 flows (not including sump return) in an X formation if you are looking down on the tank:
____________________
|X-----------------------X|
| |
| |
|X_________________X| <-- Edit: This drawing isn't working how it should, sorry.

They have each PH pointing about to the opposite corner. I'll see if I can draw it up tomorrow, this doesn't give the right impression really. Then they use smaller PH to fill any possible dead zones.
My LFS uses 2 oscillating returns and 4 PH to cover the entire tank.