It all depends on what you want, for a start I dont think anyone does a 4branch manifold for the TwinSpark but if you remove your exhaust heat shied your notice that the Twiny comes with a 4-2-1 which is a real bonus
To get the gases flowing more easy you could lose the centre section from the system, this will not only add a little more pony power but also increase the exhaust note if thats your thing.
You've done a good thing going for the sports cat as it has less cells to allow more gases flow.
Basically it works like this (in theory)
A twin box system (back box / centre box) should produce more low to mid range power as it adds torque.
A single box system (back box only) should move that gains in the rev range higher up and more mid to high as it slightly lowers torque.
But in all honesty you'd probally only notice this or see it with a back to back test with both systems or on a rolling road.
Changing back boxes might make a slight different but again not much maybe 1bhp or 3bhp max, Id settle on a design you like and stick with that, if you want a little extra volume then lose the centre box. It should show in making the engine a little more free reving too.
If you haven't already you could look into improving induction.
With either enclosed or open filters, both have pros and cons.
Get some pic's of the system and tailpipe as it is now.
Hope that helps