My 8 has an O-320. I bought it that way last June. New to the RV8 world, I had visions of grandeur of picking up an 8 that met all my needs with an IO-360. The problem was last year, they were so hot, some were selling within 15 minutes of being posted on forums and websites if they were nice. I found my 8 and it was extremely well built but didn't have the engine I wanted. As others have said, the price was 20k less than most 8s because of the engine. My guru at my field convinced me that I could buy it, sell the 320 and put the engine I want on and overall spend less with an excellent built plane.
Performance, the airplane flies nice, light nose, fun to fly. I do a decent amount of formation and my bud's 8 is a 200hp, and a bud in an RV7 is a 180 hp. To use his phrase, it was like I was always stuck in 3rd gear.
I looked for a motor for 6 months and couldn't find something to fit my needs. So from a recommendation of an RV4 friend, I sent my cylinders out and had Ly-con do their deal to them and put in higher compression pistons. the instant I sent them out, an io-360 motor fell in my lap.
Once I got the O-320 cylinders back, we hung them and put a different prop on. It is like a completely different airplane. Its performance is equivalent to the 180hp RVs in our group. It is an awesome flying plane, with a light nose. Happy with it, yes but wanting just a little more.
I am still going to hang the IO-360 with CS, but right now flying is good and I'm not in a rush. On the resale market, with a 320, it will take longer to sell and at a lower price.
So, after this long story, my recommendation to you is, hang an io-360, you can still keep it reasonably light and you won't regret it. I haven't ever heard an 8 owner say, man, I wish I woulda put a 320 on my plane instead of a 360.