1st. Its your choice, but i wouldn't temper with the iphones settings. Apple designed it uniquely.
2. The 4s, is really fast trust me on that one. It's pretty smooth and quick, unless you have like 70 backround apps running. (which can be closed by double tapping home, and holding your finger on the app then pressing red mark on the app.)
3. If your carrier Orange has 4g , (not sure because i am in America) It will make it a little faster on data, but battery life will be eaten somewhat fast.
4. I would just get it, and if you want to try to make it faster go for it. I suggest getting a case that extends the battery, i think you can find one for about 70 dollars. again sorry, for i know not UK currency, or carrier.