I think they will be awful if you top them a few feet down, straight across, especially the doublefiles. But is there really a way to get them to the level you want (which then will then quickly outgrow again and again)? Hard without seeing their habit. Any chance that opening them up to let you "see through" them would be OK, instead of a total beheading? So pruning judiciously as you say a piece here and there to shorten some, removing some height here and there, but also taking out some stems at the base for a more open plant over all. Would that suffice? Less dense but still tall? The cranberrybush types you could probably do whatever you want to (notice how they grow compared to the doublefiles, which are so decidedly horizontal). You could practically make the cranberrybush into a hedge, I suspect, but a looser pruning style is more suitable and natural. Whatever you do the plant will regrow and the same issue will recur in a year or two at most. I keep thinking of how many times I’ve cut my cranberrybush nearly to the ground, only to have it outgrow its space in a few short years again. And I cut it nearly to the ground because I hate the way plants break after being beheaded or cut back by half, say, and become a misshapen mess. Right at deckside, you won’t be happen with the results, I fear, of annual two-foot beheadings. Is this a wrong plant, wrong place situation, and maybe you need to move them and substitute a shorter variety?