A significant amount. When I first finished my TT swap on my vert, I ran open test pipes for a bit, and then when gaskets came in put the stock 2+0 NA catback back on. The difference in power and spool was disgusting. The difference between spinning (narrow) tires in 3rd and not doing so in similar conditions. But having a car that was still decently fast AND quiet was nice...
Even the difference between the stock catback and a still relatively small piped Borla catback was noticeable.