Syco

simpler(?) to try first.. move the coil forward or back 1/16" to see if changing the plunger/coil startpoint might help them hit a little harder. not deep enough could be making them mushy, or too deep making them slowish. I'll admit it right now too that I AM just guessing (and maybe right?). these units were mass produced slammed together, something 1-2mm off wouldnt have been noticed in production, but might be why-how your flippers are TOO weak?
no cost but a little effort experimenting with coil-plunger start position/relationship.
(hell, if it works.. sorry I didnt think of it a long time ago!)

check plunger pull angle and plunger puller angle stuff too while there, any kinking or binding in any of the stroke causing plunger to drag in its sleeve? small adjustments to where the coil casing is planted might (finally!) get your stuff working better, I hope.

it didnt occur to me either for never having the problem myself, but the startpoint depth of plunger in coil-sleeve can very likely affect strength and speed. a friends HvsV has a somewhat mushy left flipper, really hard to make the ramp on the right side..
what if my latest guess is IT? I can see how coil mech could have been planted 1-2mm off if the pf moved that much on an assembly line. right flipper no problem but that left one, ouch.
the guess is the coil needs to be 1/16 or 3/32 closer for slightly deeper plunger startpoint.
same left flipper is kinda spongy bouncy when catching too, like not strong enough.