The way I see it Plasma Thrusters plus Sail is a fire support ship since it needs PD coverage from someone else.
PD plus Sail is a patrol boat or pirate hunter. Since it's perfectly capable of operating on its own but has worse staying power in slugfests.
The problem is that destroyers can't take a hit and relay on their speed heavily to not get swatted.
Going down to pretty much just the Plasma thrusters means they are easy to pick.
A cheapest reasonable refit would be slapping a holo field on our existing cutters. With a heavy lascannons and twin sails they'd become a very good flanking snipers that are very hard to pin down, a true eldar mosquito fleet. And we have about a hundred of them to repair and refit.
So something like this :
[ ] Purple Void Jelly
-[] Sloop
-[] Heavy Weapon Batteries: Heavy Las-Lance, Weapons Batteries Las-Lance
-[] System: 2 Æthersail, Holo-Field, Reinforced Hull
?
Heavy las-lance is 8 NEP, and Las-Lance is 5NEP, Holo-Field is 3NEP
Unknown cost for Sail and Reinforced hull.
-> 16NEP baseline, potentially 11 if you cut the Las-Lance (bit more if you replace that with something else like plasma thrusters).
Overall refit costs should be pretty damn low considering the Heavy Las-Lance is the weapon of choice for our existing sloops.
Edit:
[ ] Purple Void Jelly v.2
-[] Sloop
-[] Heavy Weapon Batteries: Heavy Las-Lance
-[] System: 2 Æthersail, Holo-Field, Reinforced Hull, Plasma thruster
At the limited of the faster possible ships we can build i think.
And likely dirt cheap and quite hard to actually kill due to speed and reinforced hull.