Those aren't terrible 60's, but there is room for improvement. What are the control arm bushings? Are they solid? Are the lowers adjustable? I'm no expert, but I am trying to learn from other guys that are getting it done with 4-link setups. From the videos it looks like the car plants the tires well and stays flat, which is a great start. Have you run an actual slick and not the "street" slick?
A lot of the guys I know that run pure slicks pick up a tenth to a tenth and a half in the 60' from going to slicks over the street versions. This is with stick cars, but maybe that would make a difference.
Guys like Ray and the like would be great to talk to about suspension too, as they have it working well.