The only reason to "upgrade" the factory FPR is really in three instances:
1. When your injectors are starting to max out, but you don't want to upgrade them, so you bump up the rail pressure with the aftermarket adjustable fuel pressure regulator to push them a little but furthur. Obviously this isn't ideal and most people don't recommend it. Just buy the correct injectors to start with.
2. When you are running huge amounts of boost and the factory fuel pressure regulator can not keep up the 1:1 fuel pressure ratio. Therefore you will need an aftermarket fuel pressure regulator that can handle this high amount of boost pressure and keep up the 1:1 ratio.
3. The factory fuel pressure regulator has failed so you may choose to buy an aftermarket one however still run it at factory fuel pressure.
So as you can see, not many people fall into the second category because a lot of people don't run 2+bar of boost through their car. A few do fall into the first category because they may be on a budget and don't want to upgrade the stock injectors so they choose to bump the rail pressure up instead and obviously a few will fall into the third category.
However there is a fourth category that you will find 90% of people with aftermarket fuel pressure regulators fall into:
4. They have no idea what an adjustable fuel pressure regulator does, but they got told to buy one by a mate or read about one somewhere.
For your situation assuming that you are not running E85, your 700cc injectors should be fine for between 400-500HP therefore you do not need an aftermarket fuel pressure regulator.