Nice bud, very nice!
It's good to see someone going to the extreme with an Almera in the U.K.
However, just because the tyres are 245 in width, doesn't mean you will have anywhere near that in contact surface.
I had a set of 245/50/15 Pirelli Sports which are regarded as sport tyres, but after you subract the gaps on the the tread, it's actually nearer to a 205mm in contact area.
If you want to get serious, get a semi or full slick.
I dont quite understand. Is that black number plate surround from a GTiR?
Have you thought about doing something with the rear bumper? I saw a theory a while ago about it acting like a parachute, and some of the honda boys were drilling huge holes in it to allow air to pass through (which looked dreadful of course). It made sense for drag cars though. Some sort of skirty thing under the bar might work better to stop the air pressure building up in the first place.