Regarding the DPF, it does not seem worth the effort to remove it. I haven't yet found a post (any marque) where some one has measured the performance results (on a dyno) before and after removing the DPF. It all seems to be anecdotal.
Depends on why you remove it. If you remove for power gains, then obviously this needs to backed-up on a dyno. But, if you remove for driveability, fuel consumption and reliability then anecdotal evidence is all you'll get.
My average journey is a 4 mile round-trip that I do daily. Before removing the DPF my Brera ran like a choked pig. Until it got to 2000 revs there was no discernible torque, it was hit-and-miss whether or not it would start 1st time or 5th time, I couldn't even take it up a decent hill at 40 in 4th. Every week or so I'd get a warning light and limp mode that would only be cured by a good thrashing (not a hardship, but not good for the fuel consumption either). I was averaging just over 25mpg.
Now I've removed the DPF I've not had a single warning light, it accelerates nicely from 1200rpm, starts first time and the same hill can be taken at 40 in 5th! Fuel consumption is up to just under 30mpg too.
Yes, I've also given the EGR, throttle body and intake manifold a damn good cleaning, sorted the leaky/loose turbo inlet problem, cleaned the MAP sensor and fixed the swirl valves. BUT, all of those (except the turbo inlet) are problems at least in part caused by carp in the system from the combination of DPF and EGR. I haven't even bothered to fully block my EGR yet as when I started to undo everything in preparation it still looked like I'd only just cleaned it.
While I fully appreciate what Old Engineer says above about the pollution side of things, at the same time it's not exactly good for the environment (people included) to run a system which self-cleans (when it can be bothered to work properly) by dumping loads more fuel into the system increasing fuel usage and hydrocarbon emissions. The alternative thrashing, for those of us that do short journeys only, is arguably worse from an ecological point of view and, if like me you don't have a decent stretch of high speed road nearby, comes with a slightly higher accident risk.
I didn't want to risk having a cored DPF and then VOSA changing the rules from a visual test to a functional one, forcing me to replace the DPF completely at a time when people would be looking to buy them quickly before their MOTs with the consequential increase in prices. I picked up a straight pipe from an ebay seller in Italy for £55 including shipping. It has all the mounts in the correct places, plus the boss for the pressure sensor and outlet for temp sensor. It even came with a new set of bolts. If the MOT centre don't know their Alfas then it's possible they may not pick up that there's not a close-coupled DPF hidden by the under-tray as it looks OEM. If they do, then it's a simple 1 hour job to drop the straight pipe and put the DPF back on for the test.