Ok, i think i have definitely solved this issue by modifying the AGPM plist.
Here is my entry for the iMac 12,2 section (basically added something close to the MacPro inside)
<key>iMac12,2</key>
<dict>
<key>Vendor10deDevice1080</key>
<dict>
<key>Heuristic</key>
<dict>
<key>ID</key>
<integer>0</integer>
<key>IdleInterval</key>
<integer>250</integer>
<key>SensorOption</key>
<integer>1</integer>
<key>SensorSampleRate</key>
<integer>4</integer>
<key>TargetCount</key>
<integer>5</integer>
<key>Threshold_High</key>
<array>
<integer>88</integer>
<integer>88</integer>
<integer>88</integer>
<integer>100</integer>
</array>
<key>Threshold_Low</key>
<array>
<integer>0</integer>
<integer>95</integer>
<integer>95</integer>
<integer>95</integer>
</array>
</dict>
<key>LogControl</key>
<integer>0</integer>
<key>control-id</key>
<integer>18</integer>
</dict>
</dict>
New log in the console :
23:25:11 kernel: [AGPM Controller] build GPUDict by Vendor10deDevice1080
This way i keep an iMac 12,2 definition (best for 2600K), and i have a fully functional PM for the GTX 580, even this time after a wake from sleep. The card finally runs as it should be.