renovation pruning usually means removing/cutting down a third of the stems evenly around the shrub. then the following year another third and the next yr the final third.

having said that I have hard pruned them when I've needed to and they have recovered.
If you were thinking of removing them then cut them hard what have you got to loose.

I have also pruned mine back hard,,when they got too leggy,and they have always recovered well..as said above,what have you got to lose ?

Yes you can hard prune them, and whilst most do respond to be cutting hard back some can take a while to get going, in this situation you could take a few cuttings and pot them up, hebe are very easy to take cuttings from, or another method which works well is to dig a small hole at the back of the hebe then bend a decent bunch of those top stems into it and backfill with the soil, place a stone on top of the soil and leave for one year by which time they will have rooted then cut from parent plant, then grub out the old hebe, you may like to cut most of the hebe down as well as doing this procedure, like I say it works really well.