not to teach anyone to suck eggs, but I would certainly recommend investing in brakes and suspension before investinging in performance - from experience I left my brakes quite late and ended up missing 8 months due to brakes just not coping, once the brembo's went on it transformed how I could use the car ... the biggest single improvement though was changing from normal (Ohlins) shocks and springs to Tien monoflex coilovers, I can now push all the way through the corner and the car feels 100% stable.
Now need to get a better set of tyres and before everyone shouts 888's .. I need a road / track capable tyre, I can't go 888's until I've either baffled my sump or fitted an accusump as I'm sure I'll simply kill the engine with massive corning G's

.
Still need to get a cage too

... (that's if I don't go the other way and break the car which I am also toying with at the moment).