Good point. That's why I don't like it when people speed up vocals--it changes the natural timbre of the performance. Especially vocally. Dance remixes really suffer from this--i'm not sure exactly if they did anything but speed the vocals up, but it usually sounds terrible.
One thing that i've noticed about tempo--in general--is that if you speed something or slow it down even by a few BPM, it changes the feel of the song lots, performance-wise. That's why I really try to settle on that tempo where it should stay that way with enough room in the tempo for space and an in the pocket feel. When some remixer is messing around with tempo, IMHO, alot of them don't have a clue about what constitutes feel or groove....I mean, technically they have groove/ rhythm, but no sense of allowing musicians breathing room to riff or feel a performance. Because one thing that I noticed is that even 3 BPM out totally can change a vibe or in the pocket feel.
Definetely a case of "don't mess with tempo". I could go on a whole separate thread of how important tempo is to allow songs to breathe and have feeling to them. Just because something's fast doesn't necessarily mean that it's a better groove or heavier--you know, alot of bands figure, "hey, this is ripping fast, that must make it more aggressive". Yes and no. Yes because technically it's faster; no because it has no space to groove on, and groove is what makes alot of things sound bigger than they actually are. Weird timing can really make things appear bigger than they are, especially when done fast--drummers can get into weird non-4/4 fills, etc.
And just because it's slow doesn't necessarily mean that it has feel or space or "epicness" or heaviness to it....if you sped up or slowed down Bonham's drums on "When The Levee Breaks", you'd totally lose the massiveness that it has. You have to pick the tempo on the basis of the material...alot of producers and/ or remix people don't put enough emphasis on tempo.