Got the seat this week and installed it today. I went with the Recaro RS-GK which is their JDM market seat. I'm slim and tall (6'2" 160lbs) but most of my height is my legs so I fit perfectly in the waist and shoulders. I mounted the seat on a sliding mount from PCI since I wanted the option of having co drivers of differing heights. With the seat adjusted for optimal leg and hand position, I have more headroom than with the stock seats.
Initial impressions are positive. I will have to wait for a few weeks before I can try them out at an event but next to the diff, this is looking to be the most influential mod on car.
Regarding my spl arm idea, that will have to wait. Maybe another arm option will spring up in the mean time as more demand for easier to use arms grows with more people building these cars.