Gemma; just for reference, and I'm not telling you what to do, but I now have 29 koi, 3 are at approx 16-18" at 4 years old, the rest vary from 4" to 10". In addition, I have about 100 goldfish, most are over 8". My pond is about 7k gallons and I only filter with a bog. I have no intention of rehoming (despite what many out there are eventually going to tell me). I don't feed for growth, typically one feeding a day in the summer and since koi typically reach full adulthood by year 3, I figure I'm not going to have any 30" fish, no matter the statistics. And since mine are not of high genetic stock, I'm not looking to see overly large fish, imo.
I figure, if you keep up the filtration and water quality, along with plenty of aeration and plants, you can keep more. Or, that's what I'm going to prove over time!