GS450 exhaust on XL125

I have a small question. I got the front part of a gs450 and I was thinking about fitting it to my Xl125
now the first part of the downtube near the flange is 32mm diameter but it looks like it gets narrower a few centimeters in.
The Xl125 exhaust is 28mm diameter on the inside.

If I was to weld a piece of 28mm tube on the GS exhaust so it would fit the XL flange and stuff, how would this effect performance or
overall functioning of the exhaust and the bike it's self?

The nice part is is that my aftermarket muffler fits perfect on the GS450 pipe.

Thanks in advance

The GS header will be double walled - that's why you can see it get thinner on the inside.

If this ID matches the XL pipe then it should run OK.