Doubling up base gaskets works just fine, do it all the time (you could 3Bond them together if you are doubtful). However you can run a plate cut from ally with a gasket on either side if going for extreme height change, which you won’t be.
As an experiment yes it is easy, but the change in compression alters two variables (but you could machine the head as you suggest, however you can’t go back then).
Another temporary experiment in the bad old days was to file the front of the piston to give more exhaust duration without increase in transfer. Problem is newer pistons have the ring close to the top of the piston & removing any metal from there is a bit dodgy & while you might get away with it for a dyno run to indicate if you can raise the port, I wouldn’t want to run the piston this way for very long at all.
This is why tuners like E.Gorr start to look favourably cheap as you are paying for their R&D time.