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Under saddle transducers, also called USTs, are most often piezo units. They generally use either a ceramic material, such as lead zirconium titanate, sometimes called PZT, or poyvinyl-flouride that's been heated and stretched to make it both piezo- and pyro-electric. Although the original piezo materials were crystals, such as Rochelle salts, we don't use them in pickups for the most part, since the ceramics and Kynar, the trade name for PVF piezo. Each type has some advantages and some drawbacks.
Usually the piezo material is sandwiched between pieces of metal foil or otherwise housed in a metal holder, such as the one that Takamine uses. Mechanically, they don't effect the contact between the saddle and the bridge too much. The ceramic is pretty hard, and the plastic is quite thin, like a layer of Mylar. Unlike a magnetic pickup, these output a signal in response to pressure changes, and there is no motion required beyond the scale of molecular dimensions. either way I don't think the piezo unit 'costs' much in energy transmission from the string to the bridge.
Obviously, a badly fitted UST can be problematic, but you usually notice the effect far more electrically than acoustically. It's a drag when one or another string just doesn't put any signal into the amp. USTs need to be _very_ carefully fitted and installed if they are to work properly, and it's been my experience that you can get away with more sloppiness in a plain saddle than yo can with one with a UST under it. The USTs don't weigh much, an since you cut away a little of the saddle to put them in, the change should be negligable.
The I-beam is a top transducer, sensing the motion, or, more likely, the accelleration, of the top. It's like the 'dot' transducers in that respect; the K&K is another. Some acclellerometers use a mass to help 'tune' them, and to give more bass response, sandwiching the piezo element between the top and the mass.
The I-beam does add some mass to the top at the bridge saddle. I did not weigh any of the ones I've installed, but they aren't too heavy; nowhere near as massive as the bridge. They could change the acoustic tone a bit from the added mass: I'd expect in the direction of a little more bass, but I would not expect too much of a change.
All piezo transducers are, electrically, little capacitors. They have very high impedance, and the input of most ams is not at all happy with that. They usually sound 'tinny' ot 'thin' unless you use a buffering pre-amp, which will require a power source. I have had some luck with piezos smoothing out the sound by putting a small inductor in line: a few microHenries, iirc.
So, the bottom line is that a properly installed UST should have little or no adverse effect on the acoustic tone. A top sensor might, if it's relatively massive compared to the bridge, but it probably is not. In the end, of course, there's only one way to tell: listen to the thing!
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