Can I use an amp before my voltage regulator to ensure I get 5v?
Leon is correct here. Let me approach your question in more detail.

The issue here is simple, for an op-amp to output 7V it needs to have a rail that is at-least 7V, so you will still need another power source to power the op-amp. In most designing for op-amp they also do no pull much power from the first source(actually dissipating this power to ground) so this is not the method you want.

What you need is something that takes power from your first source and delivers it at an increased voltage. You could step up your voltage and then use your 2V drop Linear regulator to step it back down, which they have very good noise characteristics, so if you need low noise, you just have to cope.

Now if you get a buck/boost style switching regulator you can step it to 5V from the 5.4V input voltage, all the way down to 2V input with still getting 5V out. These are nasty to layout so you want to purchase a solution that has already been done.