i'm a big fan of solid subframe bushings. it is noticeable on the street and on the track. as for what it equates to in lap times- its probably not easily measurable as it plays into overall dynamics of the car.
kinda like brakes- brakes don't make the car faster, but a great brake kit will produce better lap times through confidence, feel, and being able to be on the throttle for longer.
solid bushings are just that- you're not upgrading the suspension, you are taking slop out of the suspension and drivetrain. the entire subframe moves about 1/2" (maybe a little less/more depending on where you're looking at it) under different loads. same with the differential. removing that deflection isolates all of the movement and forces the suspension to control it.
the subframe/diff movement takes energy to move. power used that moves the subframe/diff is literally lost power that could have been used to move the car.