jesus!! my rx7 comes with 235s on the front from factory and everyone complains thats not enough rubber on the front, me included!!

ill be going over 285s front and rear by xmas on stock arches to help balance the car out

since your high speed seems to be fine you need to be careful with your balance, if your going to add canards, you need to run a decent angle with them to make up for your lack of tyre on the front, this will help with your lower speed understeer, but then id advice you extend the wing on the rear of the car, the rs500 mini gurney does help at high speed but if you bolt on a extension to that in ali(curved of course) this will increase your lower speed downforce level and balance out the canards.

you should be able to get more tyre on that id imagine, you might need to work on the arches a bit but your gonna need more tyre to compete, especially at the lower speed corners where mechanical grip is more important.

if your struggling with the wider tyres rubbing on the outside of your arches, what are the insides like, you have to remember its not a daily driver or a drift car so you dont need to worry really what its like on full lock(unless your sideways everywhere, in which case suspension needs work)

the way to get bigger tyres is to increase the width of the wheels but offset that with a high ET, ie if your running 8inch et30 wheels, move to 9inch et42 which means your wheels 'stick' out the same as the 8inch but you have big tyres on there.

good thinking on the splitter, you want it running back to at least the sump, the further you go back the better, if you can extend it to the gearbox then thats ideal.

if you want some pics of my canards i can stick them up for you for ideas.