A team member could comment better than I, but unless TypeScript adds some kind of operator overloading, supporting a new type would be difficult since it compiles to normal JavaScript. Even then, creating a performance decimal type without native code support might be difficult.
Yeah performance is another issue. But providing a type that compiles to a defined type that implements decimal representation would be very useful. GWT pulls off something similar by compiling the Java BigDecimal to a JavaScript equivalent.
