This is very strange... less inductance in the plate choke implies
less inductive reactance, thus needing a lower capacitive reactance
(MORE capacitance) in the "plate" capacitor.
Does the Tx perform well otherwise?. Or maybe you are fooled into
thinking you've found resonance, but it is at an harmonic or spurious
oscillation?.
If you've not done it yet, better replace that choke. Shorted turns
mean not only less inductance but also degrade the quality factor, may
introduce a significant power loss (and overheat too).
By the way, 2.5 mH is a common value, but 1 mH or less may be more
than adequate even for 80 m, and with less spurious resonances above
that. It is even better if honey-comb wound (less self capacitance) or
split in sections.