I remember reading about it in an article by Olin Shivers (the history of T, at http://www.paulgraham.com/thist.html ).

After reading the wikipedia page about it, it doesn't seem to be available for any common platform.

How would it perform as a system programming language today (compared to C) ?

It would work fine. It had bit-level operators, easy assembler integration, simple macros. Decent looping constucts after the first version. Everything was an expression which makes lots of things easier and a good optimizing compiler (rember Wulf?). It did have a major quirk: you had to be explicit on addresses vs. values. To get a value you put a '.' in front of an address. Used to drive some people crazy, but I never had any problem (much) with it.