I'd run the numbers on something like desktop dyno. It won't be dead accurate but it would give a pretty good idea of what you could expect a) from a head swap or b) from a rocker swap.

If you do a rocker swap its a complete cash outlay with virtually no return from your old stuff. You still have to pull the heads to grind for pushrod clearance.

A head upgrade would yield a return on your existing heads. All your current hardware can be moved over to those.

FWIW I have a chart of a bunch of different heads and the difference between the stock stealth and ported stealth is somewhere between 80 and 100 HP at .500 lift. Can you hope to get even close to that with a rocker swap ?!?!?