Taking 5' off might spoil it, that's probably too much. Spring-summer shearing gives it time to fill back in a bit before winter.

Prune/shear in late winter or early spring. If you want that much off the top, take about 2' a year til you get it where you want it. Remember the top should be narrower than the bottom or else you'll shade the bottom and the lower branches will die.

Evergreen hedges are best pruned in spring and summer. If a flat surface is desired, they need to be done in spring and summer. Not much is likely to be gained by breaking a 5' height reduction into 2' increments a year apart. Any sprouting that occurs in response to each 2' increment will be seen primarily near that cut, only to be removed when the next cut is made a year later.

"RENOVATING CONIFER HEDGES

Conifers are ideal plants for hedging in many ways. However, they may pose problems if they are neglected. Initial training followed by regular trimming is especially important to maintain a dense, even surface. If neglected, the usual result is a bare, open center with a fringe of foliage at the extremities. The hedge then becomes prone to splitting or collapse in heavy snow or strong winds. A few conifers, especially yew (Taxus), respond well to hard pruning, but many do not readily produce shoots from old wood. Branches can be tied over to conceal holes, but in the long term, replacement will eventually become necessary."

--Brickell/Joyce, PRUNING & TRAINING (DK)

Thanks, bboy, although that is not what I wanted to hear.
I just bought this place last summer and have no idea how old the hedge is but, as you pointed out, it's been neglected.
I suppose I'll just have to pick away at it.

Does anyone know how tall these things get in zone 5, if left to their own devices?