Usually around 10%. More if you use a huge bank of capacitors, but that costs even more yet, like over 10G.
That link was a no-holds-barred conversion. It's possible to use some not-so-good components and get the same sorts of things done, but it will be more work. If you like working on cars and aren't afraid to experiment, you might be able to do it the hard way.
Here's the place to start on a lot of these questions:
From what I gather from all this, if you're doing a conversion to save dollars then you need to stop right now. It's possible, but not for the type of car you're looking at, with the capabilities you're looking at. If, on the other hand, you're a gear head who wants a really cool car, then go right on ahead.
The caps will give you two things: More power from Regenerative Braking
, and a huge kick in the seat when the light turns green, assuming your controller can handle it. Batteries don't like being run at full current rating. The less current you draw from them, the longer they last. That goes both for that charge and for the overall life of your batteries. Start doing brake torques and you will trash your batteries unless you have a capacitor bank that can handle it and still protect the batteries.
is only one of the features you get with AC, and for some reason it's what everyone focuses on. For me, it's the potential higher efficiency and a torque curve that is literally flat from zero until it gets over half of red line. I would have thought you would have grabbed that right away with your particular car.
I gave you that link because his stated intent was similar to your stated intent. I would look closely at what he did, and try to figure out why he made that particular choice. Then decide if that particular choice is compatible with your situation. You could still go with AC motors and spend a whole lot less than he did. You could use lead-acid batteries for now, and still use a capacitor bank. Your performance will be less than his, but you would still get a car that can break the speed limit when you want to, even though it won't go as far on a charge and won't set you quite so far back in the seat.
Whatever you do though, remember that batteries WILL wear out, and no matter how much or how little you spend on them, the charger and battery management system will either make the most out of them or trash them in a few cycles, depending on how well you paid attention.