So anyway, as she went back to her flat the other evening, she found her door open. Walked in (not much of a situationally aware person, eh?

There were a number of weird details about that attempt (I call it attempt because he'd just entered and didn't have time to take anything), one being the time at which it was commited (10pm on a Friday), the other being the fact that the flat was on a 3rd floot, rather than ground level, but the one serious concern is the way he gained entry: through the main door, without using force.
There are three ways burglars gain entry here: prying the door open, carding it, or finding an open access (such as an open door or accessible window). [Notice that lock picking is not part of their repertoire]. This is according to a friend of mine who is a police detective in charge of organised fraud and property crime (coincidentally, he lives in that same building).
Since the door was in one piece, he'd obviously not used a pry bar (which likely wouldn't have worked anyway, the door being steel-reinforced), so I looked at the frame and door to see if he'd carded it (assuming the door hadn't been locked), but apart from carding being rather difficult on that frame, there were no scratches or marks of any kind whatsoever. And we know the door hadn't been left open (unlocked, perhaps, but not open), so having eliminated those and other possibilites (such as gaining entry from the roof), we concluded that he opened the door in the easiest possible way: using a key.
The lock on that door was a Tesa T-60, keys for which can be copied pretty much in any hardware store. The fact that my friend had given the keys to her flat to a cleaning company a few days prior makes us think that one of the cleaners must have made a set of copies before returning them.
The first measure we took was to replace the cylinder with a Tesa TX-80, with restricted keys (which don't look that easy to machine either). The way it works (as you will know) is you get a card with a code, which you put in a safe place, and if you ever need a new copy of the key, you show the code to the guy in the shop, who then orders the new key from the manufacturer, who sends it to them and you come back a few days latter and pick it up. She will also be installing an alarm to scare them off next time they try (probably by gaining access to the roof and from there to her terrace).
Now, all that was the background for my questions, which are:
* How effective are restricted keys in stopping people from making unauthorised copies?
* How much is home security improved by having restricted keys, assuming proper key control of existing copies and the authorisation code?
As usual, thanks for your answers.