Mark,
where is your adventures spirit, the urge to live dangerously?
But seriously, your advice about backing up is correct, move a copy of your Logic app to some other place so you can retrieve the original tiff files when you need them.
I'm using the customized images now for for more than a year in L8 and now in L9 and couldn't see any side effects. The only side effect I'm observing is an increase of productivity and fun when working with Logic professionally 12-14h a day.
Here is a link to a longer thread with links and images from other Logic users with their own GUI mods. After all it all comes down to personal preferences and taste. If it works for you, use it - if not, ignore it.
http://www.logicprohelp.com/viewtopic.php?t=44387
Have fun