I've caught fish no longer than my little finger on flies big enough that it was hard to imagine how the little sucker managed to get a-hold of the thing. You are safe with a 5wt line.

There are some considerations underliing: WF lines are a problem false casting once you get the head out of the tip top. The running/shooting diameter line will not control the head and timing becomes critical to a fault because you have to be pulling the weight of the head at precisely the right time to load the rod. Add wind and you are toast assuming that you lined down to a 4wt or 3wt from 5wt, and much beyond a breeze will cause enough trouble with the 5wt line which isn't all that heavy itself. So if you want to underline and still have control on casts longer than 40' the DT and TT lines will be less frustrating, and of those two the TT would be the least trouble since the butt of that taper will always be heavier than the tip.

If you are not fishing far enough away to worry about getting the head of a WF line out of the tip top, then go with a longer/lighter compound leader not a lighter line. There are also furled leaders that act much like a triangle taper fly line and will help the leader act much like a very light fly line rolling out, and that will work fine with your 5wt

Much of the pleasure of fly fishing (for me at least) is the sensation of the rod working with the line while casting. When you under line your rod much of that enjoyable sensation is lost. The lighter the line the greater problem with wind spoiling your efforts in general and more so with underlining in particular.

Underlining is the kiss of death for a spey set-up.