Had a bit of a muck about with LTspice, but can't seem to work out a way to gang the DAC outputs together to get more power - the balun approach doesn't seem to work*.
However, what I did find is that the filter supposedly on the module is a bunch of junk - certainly not suitable for driving a notionally 50ohm antenna. A better one might be:
together with R4=51ohms, R5=no fit, R6 (Rset)=1.95kohm. Oh, and a series DC blocking cap somewhere.
LTSpice then gives the output power around 20mW (+13dBm) and rolls off at about 70MHz. When I next get into work I'll swap all the components over, and stick the output on a power meter and see...
(*update: oh hang on, maybe I've just worked out a way to get ~38mW (+16dBm) out?)