Well, your goal of 10" at 600 yards is most certainly comp level. Extremely good comp level, at that. None of the instructors at my NRA High Power Service Rifle clinic could do that, if we're talking prone with a sling and a service rifle.
I guess you're on track with getting the flat-top receiver then, so you can properly mount a scope. I'm not sure what size silhouette you were shooting, but a rack grade AR15 should be capable of hitting a human silhouette at 500 all day. So, not knowing what size silhouette you were shooting, I'm still going to danger that any further spending would be better spent on training after you put on the scope and FF tube as that should be a pretty capable platform to start with. When you get 9/10 or 10/10 hits on the target you were previously getting 2/5 on, then it'd be time to start looking at further upgrades to the AR15 itself.
Sure. I think an honest assessment is in order, however. The goal you set is a realistic goal, but it's going to take some real investment on your part in terms of training and the rifle. For 10" at 600 yards, you're definitely going to need a match grade barrel. If you're talking about doing this in the prone with a sling, you're also going to need a nice quality sling (I have a Turner's that I love), decent shooting mat, and a high quality stiff competition shooting jacket. If you're not looking to do this in traditional position shooting, and instead from a bench....then that will cut out quite a bit of the skill and specialized equipment necessary. Either way, it's still quite a bit more involved than shooting Chucky Cheese balls in a river.
Like everyone has said, it's definitely a goal you can accomplish. My input is that you still need to do an honest assessment of what you're actually willing to put in in order to accomplish it. We talking lots and lots of hours practicing to be perfect...not just practicing to do well or just shooting something for fun "just to do it."