Are you adding valves to a stock or an aftermarket (thus more aggressive) muffler? Adding valves to a stock muffler is a waste since it's the quietest muffler really can get.
The Gintani (or any other valved system) offers the best of both worlds. When you need to be socially acceptable, you can close the valves & get stock-like exhaust noise levels. AND then when you want everyone in a 5 mile radius to know you're ripping on your M3, you can open & the valves and let the S65 sing.
The difference between building your own & buying a pre-valved system (Gintani, Evolve, iPE, etc), is that the systems are designed to sound good whether the valves are open OR closed & with minimum drone.
By fabricating your own exhaust, you run the possibility of making the exhaust sound like crap or unintentionally creating drone.