My business is automobile wheels - the factory ones that come originally OEM. However I'm around aftermarket wheels all of the time and have a lot of friends in the game, so I know a fair amount about this topic.
I REALLY wanted to go with 24" wheels for my Fisker. However - the bottleneck in the custom wheel game is rarely (if ever) the wheels - but rather - THE TIRES. You see, you can find plenty of people to custom-make any design, fitment and offset of custom wheel you could ever want. But who can make you a CUSTOM TIRE? Because tires don't come in every single possible combination, and you need to have specific-sized tires - especially on cars from the last 20+ years because of systems like speed sensors, ABS, an ASR. Complicating matters is the fact that the Karma is one of those truly rare cars - that has tires that are staggered front to rear - in OVERALL DIAMETER. Yes, there are many cars with staggered width, front to rear. But much more rare is to have them staggered in height, and the Karma's rear tires have a larger OD (meaning, they are taller) than the fronts. And you need to respect that ratio, lest you start triggering codes and/or other problems when the vehicle starts getting readings that it doesn't like, comparing individual wheel speeds with each other. It can throw a light. Or worse, it can trigger your ABS or ASR, thinking that one wheel is slipping when it's speed doesn't match the other - or at-least, what it 'expects' others to read.
So the problem is - finding 24" tires that are in the appropriate ratio of staggered height. They just don't exist. I did see a chrome-wrapped Karma in Miami that had 24's; all four tires were the same size and that (to me) is a no-no, for the aforementioned reason. So I'm staying with 22" and having my friend Jordan at ADV.1 make some beautiful lightweight forged wheels.
Hope this helps.
~Howard, Wheels Direct llc