28CI is a very reasonable.
Only kohlers that I know of are capable of having 32CI. So it makes everyone run a kohler! If we down the CI to 28CI there will be more varity of guys pulling with different brand engines. Like Honda, kawasawki, kohler, Briggs, etc... Honda and Briggs already have plenty of aftermarket parts for them in the go karting world. So they are easy to find compare to the Kohlers at this moment. And they have been doing it for years now.
I know if I push my Honda motor to 32CI it's a ticking time bomb bout ready to happen. It's very possible that I can pop the jug off very easily and it's not safe for people who don't know how to weld up the block and it's very costly to weld it all up.
Honda stock GX390 23.8CI
Briggs 342cc Stock 20.9CI
Kawasawki 352cc stock 21.4CI
Honda Clone 420cc 25.6CI
Honda Clone 440cc 26.8CI
Kohler CH11-14 24.3CI
Kohler CH15-16 26.0CI
So basically what I'm trying to say, there's room for improvement with all of these CI on the motors I have above.
28CI is where I think it should stand. Just my opinion!!! it still will make plenty power! And still not have tons of money in it!