A roll bar is the least of your problems.
Car Transport - International Shipping Company | CFR Rinkens Worlwide shipping company.
You can contact them for an online quote. They will only be able to give you the cost of shipping, not all the fees you will have to pay once you get your car to Japan. Here is something I took form their site as far as what you will need to satisfy Japanese customs (not licensing/shaken fees).
MOTOR VEHICLES
Duty-free for foreigners establishing residence
Must not be sold for two years after date of Import Permit
Must meet import requirements
Documents required:
Original Passport
Alien Registration Card
Form C 5360-2 or 5360
Original Registration
Engine & Chassis Number
Purchase receipt
Proforma invoice
Drivers License (copy)
Car Manual
Documents stating vehicle has been in owner's use and possession abroad
Affidavit stating that you will not sell your auto while in Japan
NOTE: Foreign cars must be modified to meet Japanese emission and safety regulations; charges are very high and for customer's account
If the above requirements are not met, import duty will be assessed at 5% of the depreciated assessed market value.
From this it looks like your car will NOT be duty free (you are not setting up residence). It sounds like you will be hit with at least a 5% fee. Since it sounds like you built most of your car yourself, you might need to do some extra explaining / filling out more forms. I would suggest contacting the nearest consulate or embassy of Japan in your area.
First, your car will take 16 days to more than a month to get to Japan from the time it gets on the boat. Since you are in the middle of the US, you wil need to get your car to a port, meaning you drive it, or pay to have it put on a truck and carried. The Pacific, its big. Once you get your car to Japan and off the boat, now you need to go get it cleared through shaken (inspection). If it does not meet standards, and it seems like you are worried about emissions, you will not be able to srive your car until you solve the problem. It is not easy to buy tools or parts in Japan yourself. There is nothing on par with NAPA, or any of those stores in Japan. If nothing on your car needs to be fixed / changed, the shaken will cost you about 700 USD. You are only going to be using it for three months, not the two years the certificate would probably be good for.
You will also need to find an insurance company willing to insure you and your car. This will be expensive and take a lot of time. Cost is in part based on engine size.
You need to rent a place to park your car, rented or not. This might be included in your rent, but it might not. Parking a car runs around 300 USD a month.
You will need to pay road tax. As far as I can tell this is based on engine size. I have no idea what the rates are, but it is about 300 USD for a 1.5 liter engine car.
Who knows what you will need to pay to get plates for the car.
Fees like insurance, road tax etc. are paid on a yearly basis, and you get a refund if you don't use all of it or change companies. However such refunds can take months to come back to you. This means you wil need to have an open bank account in Japan and a representative, also in Japan, who can conduct banking issues for you.
See why bringing over your own car is a huge pain? All the time it would take to get it road-worthy would take days and days of your free time (Saturdays). The DMV is not open on weekends, meaning you would have to take time off work to go in, if you would. A simple transfer of ownership in Japan, something that in the US can take an afternoon, takes anywhere from 4 days to a week. Throw in the couple thousand for shipping, import duty, the even grand it will cost to shaken the beast, more if you have to fix it, insurance costs, road taxes, parking fees, then shiping it all home again, changing back anything you had to alter, and the costs really, really, add up.
@Renting, I don't have the time to search the web for Japanese sites with RX-8 rental info, maybe someone else does. But lets go with this site, Hertz Japan, to make some guesses.
http://www.outdoorjapan.com/travel/hertz/travel-carrental-hertz-tarif.html
Say a sports car cost the same to rent per day as a luxury car, 18,000 yen. Not a bad guess as it is a hot car and new. A 240 day rental would cost you 40,000 USD with the yen at 106 to the dollar). Is driving in Japan for three months, before extra fees, and even assuming you can get a deal of some sort, worth about 40,000 dollars?!!!
It sounds like you have a sweet ride, but think about putting it in storage until you get home. You will only be gone eight months, and it will be there when you get home. Believe me when I tell you the paperwork that will accompany you if you decide to import your car will make life a living hell. It won't matter if you speak Japanese. If I were doing the same thing as you, I would count on spending all of my free time for at least a month to get such a car legal to drive in Japan. With eight months and a good knowledge of the train / bus system, you will have access to more things to do than you have time to see. Also, the alcohol limit is 0.0. If you go driving, you will not be drinking. Just thought I should throw that in.
To me, all the paperwork and lost time, or around 40,000 USD to rent is not worth it. Buying a car is not that much easier, and you would have to spend a lot of time getting rid of it when it would be time to go home. Think of all the saved cash you could spent on side trips, food, ryokan, entertainment...
I think that is about it. Iron Chef might be able to help too.
Feral-D, don't dispare! Mass transit and taxis in Japan are not, in my exerience, as bad as in the US. They are on time, fairly clean, and almost everyone is respectful of other riders. It is not uncommon, though not all that often, that young folks will give up their seat to an elderly person. You can buy bus and train passes/bulk tickets and once you figure out the network, you can get almost anywhere you need to go in a resonable amount of time (sometimes much faster than it would take to drive). Since you will be in Tokyo, you really don't need a car to get to the grocery store or other retail outlets. 
