this all really depends on what your trying to acheive, if we are sticking to NA tuning within fairly restrained budgets and maintaining a usable everyday engine - the standard exhaust is 2" but at some points more like 1.75", which is fine for 150bhp, and a great way for vauxhall to use the same exhaust across the entire coupe NA range, however if your increasing the input / output flow of the engine this will need to be increased, you only have to looks at a focus st 170 of clio 182 which use 2.25" as standard to realise there is a limit to what the 2" can flow, i would be worried about going straight to a 2.5" with little other mods as you will lose back pressure and really be only gaining any bhp right at the top of the rev range where this engine is not really mapped to optimum standard and losing loads of low end usable torque in the process.
i have gone a similar route to your self, with a 2" cat back with pre cat removed, vec c inlet with 3" hosing straight into the lower airbox, and a tullet manifold, my next step is a ported TB and modded upper aibox inlet, at this point i am now considering either cams or a slightly larger diamter exhaust as i feel 2" may now be the restriction, the down pipe and cat is the biggest issue as this will most likely need to be custom as you have have probably noticed as there is not much off the shelf 2.25" down pipe and cats around and i though i dont think there is any benefit of having a wider dimater cat than the rest of the system as the exhaust can only flow as much as the smallet diameter part, i also would not expect the lack of centre box to have any effect on back pressue just noise, but your milage may vary.
in my experiance the two things that have had the most effect are the manifold and pre cat removal, so its more about removal of restrictions than anything else and letting the ECU adjust