Considering getting headers custom made for my 1.5 but am wondering what difference at all it would make?

I have a backbox on the way and I already have an induction kit on the car so it can definitely suck in and expel the air so what difference would 4-2-1 headers make? Also I assume 4-2-1 would be better over 4-1 ? And yes I'm aware that these engines are hopeless

Hi,
I assume you are referring to a 4-branch exhaust manifold? I think there is someone else on this forum who has put one on a BA (but not me). You have to watch the back pressure, and not just transfer the problem further down the line.
A lot of little changes to this engine can add up to a difference you can feel, but at the end of the day this is a 1.5 engine, and a 25-year old design. Back then 89 bhp was quite good. If you want 115 bhp then that's the 1800!
Best enjoy what's there; I quite like wafting around in my 1.5, I don't expect it be a 1.8
A modern engine is different. You can get 130 bhp from a 1200 quite easily, but they don't last as long. Ask any Peugeot 308 (petrol) owner.

4-2-1 header frees up some ponies in mid range rpms, it helps the gasses to escape the cylinders better as there's a second scavenging effect because of the 3-rd merge from the 2 to 1 tubes.
The lenght of the first 4 runners dictates the first scavenging effect wich occurs at their end where they merge in the 2, their lenght dictates when the scavenging effect will be present in the rpms.
That's the same in the second scavenging effect made by the 2 to 1 but as that is further from the head so does the effect in the rpm band too.
So from the 4-2-1 headers you find some good mid range power as well as another scavenging effect wich helps a little further too.
Overall you have better breathing engine, atleast at the hot side..