just to confirm a larger bore exhaust will reduce back pressure, which normally has the effect of moving the low end torque further up the rev range, as for the tullett manifold I would recommend however temper your expectations when I first fitted it i was a little dissapointed it felt identical to the standard item, but certainly didn't lose any torque or the charictaristic z22se low end pull, however after about a week of driving the ECU obviously had learnt the new config and adjusted the fueling / timing as it now seemed to open up the post 3.5k rev range which i felt was very lacking in any urgency and restricted before, and as im sure others will testify it seems to now pull strongly to the rev limit, couple this with a bored / ported TB and the car is now more of a screamer where as before the mods I gave up driving it past 3.5k revs as it just felt dead and very unsatisfying, now all i want to do is thrash it about as it feels much more like a light weight high revving hot hatch, other little mods that are worth considering are the EGR pipe removal if you have done the EGR mod, and try and get some 3" trunking straight from a vec c pipe into the air box.
I am still stuck on the system now, I did a little research and it would seem most NA cars making about 170-200bhp use a 2.25" exhaust (clio sport, focus 170 etc) where as 2.5+ is more suited to turbo apps less reliant on back pressure to tune charicteristics of the engine, of course some will say they have had good results with a 2.5" but i dont like the bigger is always better approach and again im not looking for no compromise power pushed all the way to the rev limit so would still like to keep low end tractibility, I also feel the 2" fitted to the Z22SE as standard was a compromise so they could use the same exhaust across multiple engines so definately room, for improvement, gut feeling would say stick to 2.25" for NA and 2.5" if you plan to supercharge / turbo.
yes a full larger bore system will reduce back preassure, however if i had say a 2.25 downpipe connectiong to a 2" cat back this would increase back preassure wouldnt it? Coupled with a tullet manifold what kind of outcome would be achieved? Better torque across the whole rev range? I am the same as yourself, i dont want a track day thrasher with all the power top end so i have to revv the nuts off it. I want the power right the way throught the rev range.
As for the EGR valve, mine is still connected. I was thinking about blanking it and fitting the cheater but if it aint' broke dont fix it in my opinion haha. I do have the vec c intake fitted but the next thing i might do is change the airbox to t/b pipe to a larger 74mm bore (i think) to help reduce restriction there too.