Interesting...so if I go backI don't know how much heavier a straight pipe is than the resonator and mufflers you removed but even if they are heavier, I cannot imagine they are heavier enough that your car would feel noticeably slower. I can't imagine your car ever felt noticeably slower when a friend (150-200 lbs) jumped into the passenger seat. Why would an incrementally heavier pipe make a noticeable difference?
I think the problem you created is that by removing the mufflers and resonator and replacing it all with a large straight pipe, you actually somehow made the flow of the exhaust less efficient, if not slower than it was with the mufflers in place. Think of an exhaust like a garden hose. Make it too thin and its not going to let out enough water. Make it too wide and water is just going to trickle out of it. But, somewhere in between, is the right width hose that will allow for water to evacuate with decent pressure. I'd bet you went the equivalent of "too wide" with whatever you used to replace the exhaust bits and that's why you've lost power. Your new straight pipes, even if they're steel, just can't weigh THAT much more than the stock aluminum system with its mufflers and resonator so that your car now feels slower.
Exhaust shop guy, would he just need to replicate the width of the stock mufflers?
I think it has been a loss of power feeling instead of weight and that makes much more sense. I will have to dyno the car soon.