Any change to the restriction given by the stock airbox (less or more) on four ganged CV carbs is going to cause problems. Rejetting a single carb isn't badm but rejetting a system with four ganged CV carbs that have 6 jets in each carbs, and takes a bit of work to get them on and off, is no good. Plus, beyond just getting idle right you would really need a dyno to get full performance back. Best to clean up the stock can, paint it as desired, and use clean paper (not K&N) filters. Going single-carb seems easier, but why would Honda make it harder (and cost them more to manufacture) using four? The only single carb setups that I have seen with good idle, acceleration, and top end on GL1000s were mounted on blowers. And tuned by carb and GL1000 experts on a dyno.