Maybe I'm weird, I just don't see myself riding another bike. Unless I'm going to be on the highway for more than 3 hours I can see a more comfortable stock 550. But anything else...not really. I feel like with one bike I can focus my attention to that more. If I had more than one, I think I wouldn't give one of them 100%. That's just me, obviously.
To each his own. I think if you are going to ride one bike and one bike only, make it the best you can to suit your riding style. I've become a big fan of lighter weight, peppy middleweights -- easy, sharp handling and less mass to stop and go. With the stock 550, there was just not enough "go" for me. I initially thought of building a 592cc motor (ported head, cam, etc.) but I chose the 650 because I could do the same mods and have an even more peppy motor.
You are leading the way with the current "wave" of 650 motor builds here -- seeing that your bike is otherwise complete and your motor is almost finished, too. Maybe, Rusty Olive will beat you to a runner, but he's running a stock motor. I am looking forward to you getting this bike back on the road for a ride report! Me and the other "me, too" 674 builds are looking forward to it!