Cool project, I image you enjoy doing this kind of work.
Question for you. Did you consider buying a more turnkey GT3 wide body kit, saving yourself the effort, and using a tested aero body style?
BTW: The last wind tunnel analysis I read on the Exige proved the rear diffusor outter most fins need to be at most 40mm off the ground for maximum effectiveness. Your's look too height, assuming that's your final ride height.
2006 Lotus Exige: Avery Matte Graphite wrap, partial built motor (9:1 Mahle pistons, Ferrea Competition valves, ACT Clutch, ARP head studs), REV400, Nitron SAs, BOE QS exhaust, Ultradisc rotors, Moroso Oil pan, Circuitworks oil pump, CF Stuff (Cup Roof, ReVerie diffusor, side scoops, APR air dam, craftsquare mirrors), Worksbell quick release, Odyssey PC625, Morimoto Xenon HID, BOE Rear Oil Cooler, Difflow Aeroplates, BOE Swirl Pot, DaviesCraig EWP, Reversed Heater Matrix