Yeah its been done... basically what I've seen is cutting the plenum off with a bandsaw or similar, then port the runners, and weld the plenum back on. The only thing is, the results are very minimal, especially with porting the upper. The reason being the problem with the stock upper is the constriction in the throttle body tunnel due to the two bolts that "pinch" it.
You are better off porting/polishing the lower intake (gasket matching). Even then the dyno results I saw were 10HP peak...not bad for free I suppose.