Intercooler (and pipework), ECU. I looked at it and decided I'd need £1500 for a reliable 50bhp increase.
Drove a boosted 5 on track on Saturday, only running actuator pressure for reliability (5psi) - it solved the 'deadly slow' issue, but took a bit to get used to the power delivery and (relatively) poor throttle response. Not sure I liked it. Had MUCH more fun in a standard 90bhp UK model, thrashing the shit out of it and bullying everything apart from a Radical SR8 in the corners, lol.