autoholic

Hey guys, a while ago when i modded my car with m5 exterior parts i decided to add a bit of sound to the exhaust. i am still a student and studies in australia and my car is in indonesia. Indonesia doesnt have alot of mod parts for e39s. you usually have to order and wait. but because i was only going back for 2 weeks and was desperately wanting a bit of sound, i went to some exhaust shop and changed my resonator with the ones they made (custom made). So the problem is, before i installed this exhaust, my car runs nicely and the power is awesome (btw its a 528i). but after i changed the resonator, there was a dramatic drop in power, the car is like a snail and doesnt want to go to high revs. so i decided to change it back. but after they welded back the originally exhaust, the power was back BUT it feels as if its not as much as it used to be. i didnt do any dyno test at the beginning or anything so its very hard to have hard facts whether the power as dropped or not but thats what i feel, a slight drop. could this be that the person didnt weld my exhaust back properly? or could this be from the result of cutting the exhaust and welding it back that might have made the exhaust a centimeter longer or shorter from what came out from the factory (i am clueless about exhaust so i dont know whether this could make a difference ). does anyone here know what could be the problem? thanks guys hoping to fix this problem when i go back this break.