Compilers need to be close to the OS
My experience (a while ago now) was that Sun's compilers worked much better than gcc when compiling for Solaris - which is to be expected. With higher end SPARC kit there will be API available to the compiler which will enable the OS to place stuff around the system in ways which speed everything up. In effect it is, and should be, possible for the OS and compiler to help each other out, and Sun's compilers have always been good in this regard.
Is Sun's Studio the silver bullet/Trojan horse which will get all the GNU/Linux developers onto Solaris? Of course not. Is Sun removing obstacles for software developers (both free and commercial)? Absolutely.
There is a big body of commercial and free software for which GNU/Linux is the tier-1 platform, and Solaris/SPARC/x86 is tier-2. If the result of this move is that Solaris moves to tier-1 alongside GNU/Linux, Sun have their desired result.