We car topped our TI on top of our Yukon Denali for several years without too many issues, however Chekika is correct if you are mostly in salt water you pay a big price in the life of your car (we had to replace the roof on our Yukon because I was careless and allowed scratches to get out of hand). I finally did buy a cheap Harbor freight trailer (about $200 bucks, I did however have to do some mods to get it to work with the TI), and that works well, and is much quicker and easier to load and take down. However we have I think the same problem as you at our other Key West (FL) house where there is no practical place for our trailer so when we go to our other house I still have to car top the TI (plus trailer parking down there is impossible to find and very expensive ($2.50/hr)).
I just looked and you can buy aftermarket hitch receivers and put them on your self (they just bolt on in under an hour (doing the work yourself) and come with good instructions typically, you only typically just need a socket wrench). Here is an after market one I found for around $120 bucks in the US (I'm sure you have comparable mail order places locally) http://www.autoanything.com/towing/77A3 ... ingid=274x
We also bought a T-bar that fits into the hitch receiver, we bought this one from Amazon for around $90 bucks ( http://www.amazon.com/Extend-A-Truck-94 ... tbar+hitch
), again I'm sure you can find something local mail order that is comparable.
Here is a pic of our Denali all loaded up with the TI on the roof (it's about 300 miles to our other place so the boat needs to be pretty secure).
The reason I am advising to use the Tbar is the rack can hold 500 lbs securely and is what carries the majority of the weight (the complete TI weighs about 200 lbs, and the hull alone weighs around 100 lbs).
It sounds like you are already carrying kayaks on your roof and probably have already fitted roof racks to your roof, I see no reason they couldn't be used to hold the TI also as long as the receiver hitch is in place and will be supporting most of the weight and giving you the necessary structural integrity. Of course adding a V shaped bow strap is also highly recommended to keep the bow from sliding side to side.
We have been hauling kayaks on our roof for many years (at least a couple hundred thousand miles (we are campers and travel all over with a camper trailer in tow and kayaks on the roof). I have tried many different roof rack systems and this type setup seems to be the best and the easiest to load and unload.
I'm not a big guy at all and also have a bad back (who in their 60's doesn't (LOL))
There is an easy way to load and unload a kayak onto a car and a hard way, I'll describe the easy way that I use (I've used the same method for about 4 yrs now every weekend).
Basically what I do is I unload the boat completely taking everything out an making sure there is no water inside, just the hull weighs around 100 lbs and you are never lifting any more than half the weight. Actually because the TI is so long this is to your advantage (it's actually easier to car top than an Oasis (we used to have an Oasis)).
What I do is put the T-bar onto the hitch receiver (after loading all the AKA-bars, seats, gear, etc into the back of the car). I then center the boat up behind the car (on it's scupper cart), I then pick up the front and pull the bow up along side the car (about even with the rear wheels). I then place an old carpet under the stern, I then lift the bow up and hoist it on top of my head (your only lifting about 50 lbs so it's not as hard as it sounds) I then walk back hand over hand under the bow so I am now behind the car, I then step sideways to get the bow over the T-bar. Once on the T-bar the boat slides easily side to side I then center the bow on the T-bar. I have a couple clamps that I made up to help keep the boat centered (I had it slide off sideways once and it scratched the side of my wifes Denali (that was a bad thing). I then walk to the back of the boat and lift the rear straight up (remember your only lifting half the weight). I then slide the boat forward to it's final position. To have something to grab onto while pushing forward I usually I just put the scupper cart into the rear set of scupper holes and it makes a great pushing handle.
If I'm going to have the boat on the roof for a few weeks in the hot Florida sun (we typically do) I slide in a couple pool noodles along the sides of the hull to help even out the load and distribute the weight (not necessary for short distances). I then put the AMA's on the roof (one on each side) then strap the whole works down. Like I described earlier the T-bar is the main support and the hull and AMA's are strapped to the T-bar, then strapped to the roof over the front roof rack. If we are going any distance or on the highway I also tie a V shaped strap to the bow to keep the bow from sliding from side to side. We have gone thru high winds and storms (30mph plus winds) with this setup many times with no issues (free boat wash, ( LOL)).
We have at times hauled multiple kayaks on the roof, when we do that we strap the AMA's on top of the TI (we don't put them up there until after we have loaded the TI on the roof). Again the Tbar in the back even with multiple boats is what supports the majority of the weight. It's probably a good idea to have a flag on the back of the boat because the back of the boat is at least 4 feet behind the back of the car. As long as you remember it's there and park carefully it shouldn't be a problem (we have had a couple tree incidents backing into parking places ( LOL).
I hope this helps you make a decision, you won't regret buying a TI, it's a great family boat, nothing on the market compares to it. We use ours as both a kayak and as a sail boat, and ended up selling all of our other Hobies because we just never used them anymore, both my wife and I now prefer to take the TI kayak (without the big sail and AMA's) out rather than two separate kayaks when we go river kayaking, it's way faster than pretty much any kayak out there (including all the rest of the Hobie fleet) and holds a lot more gear.