Help efficiency, no. Turbo mufflers were long ago shown to be better. But your concern is sound. Glasspacks will sound cool, but will change tone (worse) over time. If you had been in Illinois, I recall they are technically illegal. If I were you, I'd ask about what turbo-style mufflers guys use these days in relation to good sound. They will stay consistent. The original '66 GTO had a great idle with an OEM muffler system. Assuming you have a pure-stock motor with original cam, it probably sounds more like a motorboat right now. There'll be only so much you can do, as you have found. I had a stock '71 326 LeMans awhile back with turbo-style mufflers and that sounded pretty good. Too bad I don't recall the specific brand/model. Others may have more helpful recommendations.