Does adding an onboard preamp to an acoustic guitar make a noticable difference in it's find quality unplugged?

Some very nice high-end guitars come with onboard electronics as an option.  My Martin DCPA is not so high-end (2nd cheapest solid-wood US Martin when I bought it) but sounds fine plugged or not.  For practice and lessons, I'm usually unplugged.

I don't think anyone changes the materials or bracing of individual instruments based on whether they were ordered with electronics or not.  Maybe in a custom build?  You'll have to get someone else's opinion on that...way above my price point!

So, if the installation is done properly, any effect is probably going to be minimal.  If your guitar is good enough to worry about a small tone change, it's probably worth it to get a qualified luthier to do the installation.