Docster...welcome to the forum. You've started quite a number of threads asking questions that have been discussed at great length here. The best thing you can do is lurk on the forum a bit, and drill down to the individual sections based on the info you want. Another great place to look is in the forum sponsor section. Go to their websites and see what they have to offer. For example...you asked about rail extensions.
Their are several options...PWS rail system for example. Pros and cons are of course a matter of opinion. Adding an extension provides extra rail length/room (obviously) for attaching accessories, but it also ads weight at the front of the weapon.
A forum sponsor...for example Midwest Gun Works has a whole section just for FN products...to include rail extensions.