A 156

Assuming that you qualify for the feat, you can take it, even if it's because of a magic item.

You just lose the benefits of the feat if you ever stop qualifying, like for example taking off a magic item that is letting you meet prerequisites, but would regain use of it again as soon as you put the item back on.

So for instance your 19 dex and +6 from a magic item gives you the needed 25 dex. Though if you ever take off the +6 item you stop being able to use the feat until you get your dex back up over 25 again.