Don't Be silly Davey, those tube would be too large for the exhaust.
Mild steel should be fine but I hope you intend to triangulate the frame a bit more?
Are the tubes 1.5mm thick or thicker, don't have to answer if thats a trade secret.
I'm thinking that the long straight tubes would flex a bit under power.
From what I have heard, weight isn't a problem for speed trials, in fact it's quite the opposite.
I have heard that some guys fill the frame tubes with lead as they need the weight for traction at high speed.
Light weight is good for acceleration but at high speed it's easy for them to get wheelspin.