will welding pipes make a diff?

[darkthoughts5000]

so im thinkin about gettin the paccesetter headders with y pipe (ceramic coated of course). but ever since i got my cat-back put on, a week later black s#it has been leaking from where the mechanic clamped the pipes together (cat-back installed a few months ago). when i get the headders installed, i was thinkin about havin the "new" mechanic weld all the seems together from the headders to the muffler. does anyone think doing this will make any difference in performance? im mainly wanting some better low end torque and horse power, and someone at work said that welding the seems would probably help out a lot since the leaking seems are most likely cutting back on my back pressure and in turn hurting my horses and rear wheel torque. what you guys think???