The car runs great right now, anything else you do is going to impair drivability to an extent. Higher flowing heads will help your top end, but hurt the low end. Based on where you've been liking your shift points I doubt you'd really enjoy having an engine with high-flowing heads.
The alternative is more cubic inches under those heads - which will bring more torque into the low end while taking advantage of the higher flowing heads. Then you're also likely looking at changing the cam to make it a better fit for more cubes, and more cubes means more exhaust gas which means headers, new exhaust, etc.
You've got some of the best factory heads on a stout short block that runs very close to how you want it to. Unless the valve guides are leaking oil, I'd cast another vote to leave them alone. I'd still port and rebuild the other x-heads you have so that when your guides DO start to leak or you have other issues, you'll have replacement heads you can swap right on and keep right on driving.
