I have a very overgrown small leaf Hebe in my front garden that I forgot to prune for ten years. Some idiot threw a lump of concrete into its centre a few weeks back, breaking all the branches and leaving it less than rotund in shape.
So, I took the opportunity to prune it hard back, taking three layered plants from its base (where the branches touch the ground and root themselves) in case my plan failed. I cut back half the branches quite radically and left the other half intact to see if it would flower and also to reduce stress for the plant.
Since then, it has successfully broken bud on the hard pruned branches and I shall be pruning the rest after flowering, which is the usual time to prune small leaved Hebes into shape.
Cut it back now if you don't mind losing the flowers, otherwise imediately after flowering is also a good time. Hebes can be slightly tender, so you want to give the plant plenty of good weather to make new leaf. They often suffer frost damage in late winter that can be cut out in late spring, but they are tough plants really and you shouldn't lose it.
A feed might be gratefully received if you are going to butcher it, just try not to rob it completely of leaf. If it needs a severe pruning, think of it as a two year job and do half this year and half next.