Spoke to Jamie Porter, very helpful man.

He says he can fix the steering by adjusting geo and new bushes for around £300. As a result, the steering stops chasing chambers and becomes more accurate with no center dead spot. Sounds great!

He can also deal with exhaust drone and farty sound for around £700. His system involves adding a small pipe to the main pipe (this creates a second flute, thereby introducing some sound variation), and a holmax resonator (a 'milk bottle' thing, connected top and bottom to main pipe) near the end (this cancels out the drone), and carbon tips. No effect on performance.