Infill a 5 to 6m box hedge

2 weeks ago, we got our 200+ year old box hedge trimmed professionally and 2 dead trees removed from it.  Unfortunately, we can now,hear,the,road noise a lot more and would like to plant some evergreen shrubs to fill in the gaps.  

I was told Laurels are quick growing and tolerate shade. Is this the best plant we could Use?  The hedge faces north, very quick draining soil so gets dry in summer. The Caucasica variety tolerates drought and shade so seems the right type.  Because the hedge is old, the ground is full of roots so how well will new plants grow?

I would appreciate some help?

thanks a lot