This might seem like a stupid question, but does this fit straight onto a 2 1/2 mongoose downpipe? Or do i need a new downpipe?
That's the benefit of Bogdan's work. - It's a direct fit, so if your Mongoose bolts onto the existing elbow it will bolt onto this one with no modification; even the lambda sensor lines-up where it's supposed to be.
I've seen others that have a lot of welds to make the radius and my concern is that the more welds used, the more heat went into the piece... and the more chance there is of distortion. Obviously if it's distorted it won't line-up with everything else. - I'm not a fabrication expert, but I did look into tips and tricks on welding when I was thinking about repairing my own arches (and decided the chance of me bodging it was too great).
If you're really uncertain you could have a custom elbow made just for you buy a competent builder, but I think that will cost you more.