My opnion is run the widest wheel within the manufacturer's recommended range.  For 255's, that means a 10" wheel typically.

FYI, there is finally an affordable 18x12 - http://www.racinglab.com/rota-p45r2-18x12-001.html
27lb, but what do you expect lol.
 

No part of the wing can stick out past the rear bumper, including uprights. Keep that in mind, might be hard to mount it the way you have it drawn.

$1000 shipped

or $900 picked up

Wheels are in great shape, aside from minor curb rash on the one wheel (shown).  I don't want to split them up, unless I get buyers for all four.

email: josh18_2k@hotmail.com
call/text: 503-702-1110

Ok it's official.   This thing needs to go away so I can spend money on my house :-(

1987 Mazda RX-7 Base (no sunroof)

The car is barely street legal, depending who you ask.  Still have lights and glass windshield.  I drive to/from events so I kept the bare minimum to not get pulled over.

Current weight = 2480# with aero and ~5gal gas.  There is plenty more to lose if you don't need to remain streetable.  Still have heater+blower with defroster and face vents.  Exhaust isn't too loud- won't get you in trouble with the law, but will likely not pass smog.  The doors are gutted;  Original un-cut doors included with power windows/mirrors and black door panels.

This car is very fast as is, with plenty of room for improvement. 
Took 1st and 3rd in XP at 2013 Packwood national tour.  Based on PAX, it could podium at Nationals as-is.  Cut the rest of the weight and switch to 16" slicks, and you have a serious contender.

Vid from Packwood NT -

$8500 with 17x8 mustang wheels and street tires (no slicks or other wheels)

$10k with everything

Drivetrain
-2000 LS1 - 342whp on a Mustang dyno (they read low)
 *Ronin mounts
 *Katech rod bolts
 *Improved Racing oil pan baffle (gen.2)
 *Improved Racing oil cooler adapter block
 *FD oil cooler
 *BBK long tube headers
 *Flowmaster y-pipe into 3.5" single exhaust with Magnaflow muffler and adjustable turndown
-Sixspeedinc stage 4 T56 (viper internals) with Hurst and stock shifters
-Fidanza Flywheel
-Diamond Stage 2 Clutch
-Aluminum Driveshaft
-OS Giken 1.5 way LSD

Suspension
-Custom Bilstein coilovers with a couple sets of springs
-Ground Control camber/caster plates
-ST Front sway bar with Mazdatrix endlinks
-MMR Delrin front control arm bushings
-MMR Sherical trailing arm bushings
-Three sets of individual camber links: MMR, AWR, and shortened stock dogbones
-Adjustable center camber link

Interior
-Racetech RT1000 driver seat, Cobra Suzuka passenger seat (both hard mounted)
-Sparco  Monza wheel with NRG quick release
-Flocked dash (no glare!)
-Stock tach/speedo work
-Oil pressure and temp gauges with warning lights
-ScanGauge to read OBD2 stuff in realtime

Chassis
-Custom aluminum 4-point strut tower brace
-Custom aluminum trailer hitch (for tire trailer, not a boat)
-Lexan hatch
-Carbon fiber wing with standard mounts (roof level, clears hatch) and XP-legal risers (6" above roof, even with rear bumper)
-Shine FC2000 side skirts (not installed)
-BN front bumper, super easy to remove/install (4 Dzus clips and two bolts)
-Birch splitter, stained mahogany for a touch of class ;-)  Also very easy to remove/install
-Corksport front fenders
-B-Magic rear fenders
-Vented aluminum hood
 
Wheels/Tires
-(4) 18x10.5 Enkei PF01
-(2) 18x10 FR500 reps
-(2) 17x10.5 FR500 reps
-(6) Hoosier A6's, 4 with 12 runs, 2 brand new
-(4) 17x8 Mustang wheels with some crappy street tires

yah i have a feeling it will end up that way :-/

I'm planning to make a proper for sale thread soon and post it all over the internet.  If I don't get any interest, part-out will happen.
I've officially semi-retired from autox as of last month.   My co-driver is building an XP vette, so we'll switch to that when it's done (probably 2015).

I used a Donjer flocking kit. I think it's made for doing duck decoys or something...  The adhesive application is super important. The areas with good coverage seem to be doing well, but I can rub the stuff off in  areas where the adhesive was too thin or dried out.

 For the price, it's worth doing a test piece and see if its durable enough for you.

A civic getting 2nd place I'm autox is a dead giveaway that no one is experienced.  Most, if not all, the cars should be faster, assuming equal+experienced driver.  Results really show how much of a difference experiance can make.
That bein said, don't be too hard on yourself about the autox results. I think most people don't realize how much time is really out there to be extracted by a high-level driver.  That was definitely one test where the car had little influence. Unless you're a GTR lol.