I can't at all agree with rsegal that T'au technology is superior to any other except Necrons. Let us take the example of the Dark Eldars Glass Plague:
The celebrated Commorrite Jalaxlar revealed his latest work, lifelike crystallized sculptures of Dark Eldar. The masterful pieces of work earn Jalaxlar acclaim across the art galleries of the city, for he has captured the expression of true shock and terror in his sculptures. However after a rival house attacked Jalaxlar's galleries that same evening, his laboritires are smashed apart and his secret revealed. The sculptor had isolated a viral helix that quickly turns living material into crystalline glass.
From the Lexicanum article on Glass Plague
So, they have a level of technology able to turn living beings into crystal. And that's the work of a single Dark Eldar. As a society, they've reached a level of technology able to move entire suns to where they want:
The city itself is powered by the Illmaea or "stolen suns". For their energy source, the Dark Eldar copied a technique of the old Eldar Empire. Dark Eldar steal suns from the Materium, bring them into their Webway realm, and use their energies to supply the Dark City.
From the Lexicanum article on Commoragh
They still have that knowledge as is shown in Asdrubael Vect handing a box with a black hole in it. So, to the question if they are able to reverse engineer any technology, lets consider the definition of reverse engineering:
Reverse engineering (also known as backwards engineering or back engineering) is a process or method through the application of which one attempts to understand through deductive reasoning how a device, process, system, or piece of software accomplishes a task with very little (if any) insight into exactly how it does so.
From Wikipedia article on Reverse Engineering
So, with that definition, I'd say "not at all" simply for the reason that they don't understand the physics required for this to work. It's the same as asking if todays scientist were able to reverse engineer technology from Star Trek. We wouldn't even even know where to start, we don't yet have the knowledge to understand how they generate that kind of energy, bend space and so on (yes, we might understand the basics but not how to run it stable and so on). Some parts of the technology might be able to be understood, other might speed up our understanding as we can test theories with a working example and other parts are so far beyond our understanding that we're more likely to blow it (and us) up than to understand it in any way.
Also, latest in 745.M41, they got a warp drive as detailed in this question. To the extend of my knowledge, they haven't a working warp drive but only Gravitic Drive, which they were able to reverse engineer. And for the explanation on psykers, humans were able to create machines that can calculate warp jumps without the need of psykers.
In conclusion: they might be able to reverse engineer some parts of the technology that is slightly ahead of them but certainly not technology so far ahead of them that they wouldn't even understand the principle it works on.