plug is useful b/c if you are mulching, it'll keep all your grass circulating in the lawn mower deck (the underside), which means it'll have more time to get mulch, otherwise you'll just get two lines of grass coming out as your wheels crush it. i know cuz i bought a mower w/o the plug (as is), i just took some styrafoam cut it a little big and wrapped it in duct tape (if you just shove styrafoam in there, it'll slowly degrade over time w/ damp grass, etc... experience) and made a ghetto plug.
to me, the mulching blade does work, since i put it on, i have no lines of grass and no clumps anywhere, it has high lift (so it keeps the clippings spinning in the deck to eventually get pulverized) and i sharpen my blade every time i mow (well more like i hone it with a leather strop to keep it razor sharp and only sharpen every month)
really does sound like you need more power, maybe make sure you have sharp blades, and if that still isn't doing anything for you, then i believe it's time to invest in more power.