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The "tall" ones shoot out from where you do the cut so if you want to keep it at 5', best to take it down to about 3' or less and let it grow up to the required height.
I try not to prune by more than a third. Or if I wanted to take it down drastically I'd wait till autumn. Falling all that I look on the RHS website as its pretty good. Good luck!
Its always said that mahonia re shoot from hard pruning, and in most cases they do -- however i have seen some that haven't - sometimes if they are old , not doing well , showing signs of weakness some of the hard cut stems do not re shoot.
If it looks in good health and a strong plant you probably fine to prune hard now -- april / may recommended rhs
If you have any concerns look for buds formed lower down the stems and cut to the lowest, as long as its got growth starting lower on each stem you can be sure it will survive cutting to that point
As per the other comments, I regularly cut mahonia down to below half height and it does great every time. I do try and make sure there's signs of life below the cut, even just a swelling on the stem.