If you get an upper, you basically have a strong trainer for your AR set up. You can duplicate your AR upper set up and since your using the same lower, you're using the same, trigger, control group, grip etc. If your conversion was simply the BCG and your using your AR barrel, you don't have an optimal twist rate. That's throwing off your accuracy.
If you're not looking for a trainer, it's still an option but as you mentioned, you can get another rifle for the price of a .22 AR upper. I'm a big advocate of the M&P15-22.