Looking at their website and configuring a couple different set ups if you were to say go with a flowmaster or magnaflow muffler(with magnaflow it says chambered not strait through this is a bad thing performance wise) it adds a good amount, upgrade to a 2.5" pipe, mandrel bent pipe, and stainless steel pipes instead of aluminium (just to make it a fair comparison to the big brands) and then add the clamp on tips (doesn't specify size probably 3-4" vs the 5" you get with most big brand kits) and you end up at roughly $440 before shipping (did not go through with a checkout or add to cart they might offer free shipping I don't know)
with that said you are not that much cheaper than the big brand names, an with the big names you get a trusted product that has been tried and is backed by the manufacturer as far as warranty goes. You might get this with these guys but I can not speak for them.
Is it worth the extra $100-$200 to go with something everyone is more familiar with and is more known/trusted only you can make that decision
, either way you choose be sure to let us know and let us know how you like what you end up going with