There won't be a problem. Here's a for instance...................My car has a 4-1 cast dbilas manifold, 2.5 inch stainless steel downpipe with no precat and a 200 cell sport cat, 2.5 inch stainless steel Milltek catback system, oh and a lovely supercharger! When i'm on the motorway at 70mph, I can run along at that speed with no load on the engine and the car doesn't sound any different to when it was standard. Well maybe a little lower tone in the exhaust note!
The only time that you'll notice any real difference in noise is when you are caning the ass off it and when you're coming back down thru' the gearbox. The exhaust may pop every now and again. Lots of people have done this mod and it works well.
;-)