Hello,
In a design with 2 12ax7 valves in the preamp and 2 el84 in the output section, could we expect much higher power development with the 80VA transformer?

According to the HT secondary specifications in the 50VA we have 250-0 at 0.120A, and in the 80VA we have 260-0-260 at 0.1A.
Would the extra 0.02A compensate for the difference in power between the 10 volts output and using a diode bridge to two diodes for rectification?.

Once rectified by a diode bridge, the 50VA transformer gives about 340 volts output adjusting the bias for 60% dissipation. In the case of the 80Va, once rectified by two diodes, we have about 362 volts output with the bias also adjusted to 60%. It doesn't seem like a big difference, but the sag may be higher on the 50VA.

Thanks in advance for the help.

Yes, at full output the difference in B+ is likely to be larger due to sag.
A B+ difference of 30V at full output roughly results in a power output difference of 20% (around 4W).
But higher output also means accelerated tube wear.