I think what you have would be an improvment , but no where near as good as some other designs around.
I am about to get mine done on monday , in twin 3 inch mandrels and they will continue as twins for as far as possible,maybe even right down to the cat. the idea being to straighten out the gas flow as much as possible.
You may still get some disturbance at the junction there.
Who was that Kiwi, who was offering $100 for the first person to post up before and after dyno charts.
there will be two separate cars with new twin pipes to post before and after results by mid newxt week.