CPatch,
You probably will not regret it. I have a MiSi in my Boat Paddle and it truly sounds great.
Undersaddle piezos are loud and brighter since they work off pressure changes rather than vibration. The more pressure you put on them the louder and more present they sound. The only way to get an undersaddle in a Fluke or a Flea is at the factory, since once they put the saddle on, it doesn't really come off. I have heard there is also a different saddle Flea Market puts on the for B-Band pickup, but I have never seen it.
I bought a Flea off the shelf and installed a Barcus Berry Insider piezo just in front of the bridge. This seemed to be the sweetest spot. It sounds good but nowhere as loud and present as an undersaddle.