I replaced the parts in the shift tower last night and holy cow what a difference. Its a fairly cheap and easy update. I highly recommend to anyone with a higher mileage Miata, I replaced the following:
Large rubber boot that bolts to the tunnel
The small rubber retainer boot that bolts to the tranny
The two nylon/plastic cone bushings
Two wavy springs
The pin slot bushing
And the button bushing on the ball on bottom of the shifter (mine was gone)

Well worth the money!!!!!
Had no idea how precise the shifting could be!!