As a word of encouragement Bart: I think the easiest way to kill your enthusiasm for flats is to go with a set that is too light in gauge. With the J neck on your P/J4 you may be tempted to do just that. Light gauge flats lack "fullness",or "body", in comparison, which is what you really want in a "P/flats" tone. A heavier gauge has slightly more tension but you can lower the action a tad to compensate. That Sad J neck aint gonna fold on ya. You will be surprised at how comfy they feel if you don't have to fight 'em. The Sadowsky SBF45's are excellent strings.
Another consideration is to NOT set the pup ht. too high. Again - higher set = volume - lower set = tone. Experiment w/ this some.
PS: I am REALLY looking forward to hearing about it if you pull the trigger on a set. You have a great bass for it, & if you think your rock gigs aren't the place for them - LOOK OUT sports fans. Don't forget you have a J pup sitting on the back burner bro. Make them BOTH work on those flats for ya.
B.
EDIT: As for anyone - It's just another flavour to taste.
Worth a look:
http://www.talkbass.com/forum/showpost.php?p=9739194&postcount=85 Put a right angled crimp w/ pliers (about 4" past post) before cutting excess off, leaving a very short bend that will fit into the post hole. Make sure the string's end ball is free to spin until you've wound the string tight, so there's no twist in the core. I've never had an issue when doing this. I also never string flats thru the body on a Fender.