Some things we learned while evaluating the 4350 a while back:
1) For wideband operation, replacing the RF output bias inductors with 50-ohm resistors (i.e. L2 and L3) works best (UG-435 now carries a similar statement for operation below 1GHz).
2) If you are only using a single RF output (e.g. RFOUTA+) make sure to terminate the other one (e.g. RFOUTA-) with 50-ohms (UG-435 also mentions this).
3) We also found terminating the unused RF output (e.g. RFOUTB+ and RFOUTB-) provided better performance. This was easy to do on the ADF4350 eval board because it already had those ports brought out and decoupled to SMA connectors. I see the ADF4351 does not have that, so if you want to try that you will have to install decoupling caps and terminating resistors yourself.