Haven't done a scientific study of it, but I do make blades for a living -- and absolutely sealing makes a difference (it stiffens the outers and speeds up the blade) but ithe degree depends hugely on the type of sealant, and it's thickness.
The three sealants I use most often are water-based polyurethane, oil-based polyurethane, and Osmo oil.
Water based poly has the least stiffening effect, oil-based poly has the most, Osmo is somewhere in the middle, but I really only use that on handles and edge grain, never on the playing surface.
A single thin coat of water-based poly (ie: wipe it on thin then wipe off any excess immediately) adds the least stiffness of all, and so long as you don't go nuts with removing the excess, this should be sufficient to protect the blade.
I only use oil-based poly when I actively *want* to thicken and stiffen the outer layers to get an extra bit more speed, and even then, I never put on a thick coat. At most, it's only one or two very thin coats.
With oil-based poly it's a lot safer to rub-off more of the excess, as it's considerably thicker and harder than water based poly (I find this to be true regardless of the brand). For this reason I will sometimes add two layers to a blade (the second later is thinned down with the tiniest hint of turpentine -- otherwise it's too thick).
As a side point, if you are going to use multiple coats of oil-based poly, you must *ALWAYS* rub the blade down between layers with semi-fine sand paper.
If you don't do this, as some oil-based polys tend to contract a bit as they dry, this can sometimes provide enough force to cup or warp the blade's playing surface. This is especially prone to happening if your blade's ply sandwich uses a lot of very low density woods and/or has a lower number of layers with transverse grain. (For this reason, if you're not particularly knowledgeable with wood finishes in general, it's actually far safer on the whole to just use one very thin coat of water based poly, and nothing else).
Hope this helps.
