Well sat some point during the testing the RF2126 1 Watt amp fried it self, so possibly why so little output. Back to playing and waiting with some replacement chips on order.
There are some issues with the chinese RF2126 design:
1. Voltage divider for Vpc is inadequate, especially @6V; use poti and set 180mA idle current carefully!
2. Capacitors and layout: See attached snap, I placed capacitors with values given in the data sheet. Removed/replaced this 0 ohm resistor in the stripline with a capacitor with the datasheet value (3p3) and put a small copper strip to connect output.
I achieved some 900mW @5V @2400MHz
Frank
Update: Due to some reason three other RF2126 fried. I give up with this amp ... 